Summary Prospectus by Investment Company (Form 497K)

MXPRO110

January 28, 2026, as amended and restated April 2, 2026

Summary Prospectus

Oakmark Fund

Before you invest, you may want to review the Fund's prospectus and statement of additional information, which contain more information about the Fund and its risks. You can find the Fund's prospectus and other information about the Fund online at www.Oakmark.com/prospectus. You can also get this information at no cost by calling 1-800-OAKMARK or by sending an email request to [email protected]. The Fund's prospectus and statement of additional information, dated January 28, 2026, and as each may be further supplemented or amended, are incorporated by reference into this Summary Prospectus.

Investment objective

Oakmark Fund seeks long-term capital appreciation.

Fees and expenses of the fund

Below are the fees and expenses that you would pay if you buy, hold, and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment).

¹​ Investor Class Shares of the Fund pay a service fee not to exceed 0.25% per annum of the average daily net assets of the Fund's Investor Class Shares. This service fee is paid to third-party intermediaries who provide services for and/or maintain shareholder accounts.

Example. The Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund's operating expenses were those reflected in the table, inclusive of any fee waivers and/or expense reimbursements.

Although your actual returns and expenses may be higher or lower, based on these assumptions your expenses would be:

PORTFOLIO TURNOVER

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Fund's performance. During the most recent fiscal year, the Fund's portfolio turnover rate was 63% of the average value of its portfolio.

Principal investment strategy

The Fund invests primarily in a diversified portfolio of common stocks of U.S. companies. The Fund generally invests in the securities of larger companies. The Fund uses a value investment philosophy in selecting equity securities. This value investment philosophy is based upon the belief that, over time, a company's stock price converges with the Adviser's estimate of the company's intrinsic value. By "intrinsic value," the Adviser means its estimate of the price a knowledgeable buyer would pay to acquire the entire business. The Adviser believes that investing in securities priced significantly below what it believes is a company's intrinsic value presents the best opportunity to achieve the Fund's investment objective. A company trading below its estimated intrinsic value is sometimes referred to as trading at a discount.

The Adviser uses this value investment philosophy to identify companies that have discounted stock prices compared to what the Adviser believes are the companies' intrinsic values. In assessing such companies, the Adviser looks for a number of attractive attributes which could include: (1) free cash flows;

(2) earnings that are growing and are reasonably predictable; and (3) high level of company management alignment with shareholders.

Key Tenets of the Oakmark Value Investment Philosophy:
1. Buy businesses that are trading at a significant discount to the Adviser's estimate of the company's intrinsic value. At the time the Adviser buys a company, the Adviser wants the company's stock to be inexpensive relative to what it believes the entire business is worth.
2. Invest with companies expected to grow shareholder value over time. Value investors can sometimes fall into the trap of buying a stock that is inexpensive for a reason-because the company just does not grow. The Adviser looks for businesses that are expected to achieve a combination of dividend yield and per-share growth in business value that is above-average.
3. Invest with management teams that think and act as owners. The Adviser seeks out companies with management teams that understand the dynamics of per share value growth and are focused on achieving such growth. Stock ownership and incentives that align managements' interests with those of shareholders are key components of this analysis.

In making its investment decisions, the Adviser uses a "bottom-up" approach focused on individual companies, rather than focusing on specific economic factors or specific industries. To facilitate its selection of investments that meet the criteria described above, the Adviser uses independent, in-house research to analyze each company. As part of this selection process, the Adviser's analysts typically visit companies and conduct other research on the companies and their industries.

Once the Adviser identifies a stock that it believes is selling at a significant discount to the Adviser's estimated intrinsic value and that the company possesses the other key tenets of the investment philosophy, the Adviser may consider buying that stock for the Fund. The Adviser usually sells a stock when the price approaches its estimated intrinsic value. This means the Adviser sets specific "buy" and "sell" targets for each stock the Fund holds. The Adviser monitors each portfolio holding and adjusts these price targets as warranted to reflect changes in a company's fundamentals.

The Fund may also use options, including, but not limited to, buying and selling (writing) put and call options on individual stocks and indexes, when such use is desirable because of tax or other considerations. Use of options may assist the Fund in managing distributions.

The Adviser believes that holding a relatively small number of stocks allows its "best ideas" to have a meaningful impact on the Fund's performance. Therefore, the Fund's portfolio typically holds thirty to sixty stocks rather than hundreds, and as a result, a higher percentage of the Fund's total assets may at times be invested in a particular sector or industry.

Principal investment risks

As an investor in the Fund, you should have a long-term perspective and be able to tolerate potentially wide fluctuations in the value of your Fund shares. Your investment in the Fund is subject to risks, including the possibility that the value of the Fund's portfolio holdings may fluctuate in response to events specific to the companies in which the Fund invests, as well as economic, political or social events in the U.S. or abroad and the

Adviser's evaluation of those events, and the success of the Adviser in implementing the Fund's investment strategy. As a result, when you redeem your Fund shares, they may be worth more or less than you paid for them.

Although the Fund makes every effort to achieve its investment objective, it cannot guarantee it will attain that investment objective. The following principal investment risks can significantly affect the Fund's performance:

Market Risk. The Fund is subject to market risk-the risk that securities markets and individual securities will increase or decrease in value. Market risk applies to every market and every security. Security prices may fluctuate widely over short or extended periods in response to adverse issuer, political, geopolitical (including wars or acts of terrorism), regulatory, market, economic, sanctions, tariffs, global health crises or pandemics, environmental, or other developments that may cause broad changes in market value, stability, and public perceptions concerning these developments, and adverse investor sentiment. In addition, securities markets tend to move in cycles. If there is a general decline in the securities markets, it is possible your investment may lose value regardless of the individual results of the companies in which the Fund invests. The magnitude of up and down price or market fluctuations over time is sometimes referred to as "volatility," which, at times, can be significant. In addition, different asset classes and geographic markets may experience periods of significant correlation with each other. As a result of this correlation, the securities and markets in which the Fund invests may experience volatility due to market, economic, political or social events, such as global health crises or pandemics, and conditions that may not readily appear to directly relate to such securities, the securities' issuer or the markets in which they trade. In addition, some companies may have substantial foreign operations or holdings and may involve additional risks relating to those markets, including but not limited to political, economic, regulatory, or other conditions in foreign countries, as well as currency exchange rates.

Focused Portfolio Risk. The Fund's portfolio tends to be invested in a relatively small number of issuers. As a result, the appreciation or depreciation of any one issuer held by the Fund will have a greater impact on the Fund's net asset value than it would if the Fund invested in a larger number of issuers. Although that strategy has the potential to generate attractive returns over time, it also increases the Fund's volatility and may lead to greater losses.

Common Stock Risk. Common stocks are generally subject to greater fluctuations in market value than other asset classes as a result of such factors as a company's business performance, investor perceptions, stock market trends and general economic conditions. The rights of common stockholders are subordinate to all other claims on a company's assets including, debt holders and preferred stockholders; therefore, the Fund could lose money if a company in which it invests becomes financially distressed.

Sector or Industry Risk. If the Fund has invested a higher percentage of its total assets in a particular sector or industry, changes affecting that sector or industry, or the perception of that sector or industry, may have a significant impact on the performance of the Fund's overall portfolio. Individual sectors or industries may be more volatile, and may perform differently, than the broader market.

Large Capitalization Risk. Investing primarily in issuers in one market capitalization category (large) carries the risk that due to current market conditions, that category may be out of favor with investors. Larger, more established companies may be unable to respond quickly to new competitive challenges or opportunities or attain the high growth rate of successful smaller companies.

Value Style Risk. Investing in "value" stocks presents the risk that the stocks may never reach what the Adviser believes are their full market values, either because the market fails to recognize what the Adviser considers to be the companies' intrinsic values or because the Adviser misjudged those values. In addition, value stocks may fall out of favor with investors and underperform other investments during given periods.

Options Risk. The use of options involves investment strategies and risks different from those associated with ordinary portfolio securities transactions. If a strategy is applied at an inappropriate time or market conditions or trends are judged incorrectly, the use of options may lower the Fund's return. There can be no guarantee that the use of options will increase the Fund's return or income. In addition, there may be an imperfect correlation between the movement in prices of options and the securities underlying them and there may at times not be a liquid secondary market for various options.

Your investment in the Fund is not a deposit in a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. You may lose money by investing in the Fund. The likelihood of loss may be greater if you invest for a shorter period of time.

Performance information

The bar chart and performance table below can help you evaluate the potential risk and reward of investing in the Fund by showing changes in the performance of the Fund's Investor Class Shares from year to year. The Fund's past performance (before and after taxes), as provided by the bar chart and performance table that follow, is not an indication of how the Fund will perform in the future. The performance table illustrates the volatility of the Fund's historical returns over various lengths of time and shows how the Fund's average annual returns compare with those of one or more broad measures of market performance. The indices, which are described in "Descriptions of Indices" in the prospectus, have characteristics relevant to the Fund's investment strategy. The Fund's Advisor Class and Institutional Class each commenced operations on November 30, 2016 and the R6 Class commenced operations on December 15, 2020. Updated performance information is available on Oakmark.com or by calling 1-800-OAKMARK (625-6275).

INVESTOR CLASS SHARES (%)

Total Returns for Years Ended December 31

Since 2016, the highest and lowest quarterly returns for the Fund's Investor Class Shares were:

• Highest quarterly return: 24.2%, during the quarter ended December 31, 2020

• Lowest quarterly return: -30.4%, during the quarter ended March 31, 2020

Average Annual Total Returns for Periods Ended December 31, 2025

* Advisor Class shares commenced operations on 11/30/2016. The performance attributed to the Advisor Class shares prior to that date is that of the Investor Class shares from 08/05/1991-11/30/2016. Performance prior to 11/30/2016 has not been adjusted to reflect the lower expenses of Advisor Class shares. During this period, Advisor Class shares would have had returns similar to, but potentially higher than, Investor Class shares due to the fact that Advisor Class shares represent interests in the same portfolio as Investor Class shares but are subject to lower expenses.

** Institutional Class shares commenced operations on 11/30/2016. The performance attributed to the Institutional Class shares prior to that date is that of the Investor Class shares from 08/05/1991-11/30/2016. Performance prior to 11/30/2016 has not been adjusted to reflect the lower expenses of Institutional Class shares. During this period, Institutional Class shares would have had returns similar to, but potentially higher than, Investor Class shares due to the fact that Institutional Class shares represent interests in the same portfolio as Investor Class shares but are subject to lower expenses.

*** R6 Class shares commenced operations on 12/15/2020. The performance attributed to the R6 Class shares prior to that date is that of the Investor Class shares from 08/05/1991-11/30/2016, the performance of the Institutional Class shares from 11/30/2016-12/15/2020, and then the performance of the R6 Class shares from 12/15/2020 onwards. Performance prior to 12/15/2020 has not been adjusted to reflect the lower expenses of R6 Class shares. During this period, R6 Class shares would have had returns similar to, but potentially higher than, Institutional and Investor Class shares due to the fact that R6 Class shares represent interests in the same portfolio as Institutional and Investor Class shares but are subject to lower expenses.

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on an investor's tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans, qualified plans, education savings accounts or individual retirement accounts. In some cases, the after-tax returns may exceed the return before taxes due to an assumed tax benefit from any losses on a sale of Fund shares at the end of the measurement period. After-tax returns are shown only for Investor Class Shares. After-tax returns for Advisor Class Shares, Institutional Class Shares, and R6 Class Shares will vary from returns shown for Investor Class Shares.

Investment adviser

Harris Associates L.P. is the investment adviser to Oakmark Fund.

Portfolio managers

William C. Nygren, CFA, Michael A. Nicolas, CFA, and Robert F. Bierig manage the Fund's portfolio. Mr. Nygren is a Vice President, Chief Investment Officer, U.S. (until June 30, 2026), Co-Chief Investment Officer, U.S. (effective close of business June 30, 2026), and a portfolio manager of the Adviser. He joined the Adviser in 1983 and has managed the Fund since 2000. Mr. Nicolas is a portfolio manager and analyst of the Adviser. He joined the Adviser in 2013 and has managed the Fund since 2020. Mr. Bierig is Deputy Chairman (effective close of business June 30, 2026), Vice President (until June 30, 2026), portfolio manager and analyst of the Adviser. He joined the Adviser in 2012 and has managed the Fund since August 1, 2022.

Purchase and sale of fund shares

The Fund's initial investment minimums generally are set forth in the table below. Once your account is open, subsequent investments may be made in any amount. Intermediaries may impose their own minimum investment requirements.

¹​ An omnibus account is a single account in the Fund held in the name of an intermediary that contains the aggregated assets for all of the intermediary's customer investments in the Fund. Consult your financial advisor or intermediary if you are unsure how your intermediary assets are held.

Shares of the Fund may be purchased and sold (redeemed) on any business day, normally any day when the New York Stock Exchange is open for regular trading. Such purchases and redemptions can be made directly with the Fund by writing to The Oakmark Funds, P.O. Box 219558 Kansas City, MO 64121-9558, or visiting Oakmark.com. Some redemptions may require a Medallion signature guarantee.

Purchases and redemptions can also be made through an intermediary, such as a broker-dealer, bank, retirement plan service provider, or retirement plan sponsor. Intermediaries may impose their own minimum investment requirements. Although the Fund does not impose any sales charges on any class of shares, you may separately pay a commission, a transaction-based fee or other fee to your intermediary on your purchase and sale of those shares, which is not reflected in this prospectus. You may be eligible to transact in the other classes of shares that are offered by the Fund that have different fees and expenses. Please contact your intermediary for additional information.

Tax information

The Fund's distributions may be taxable to you as ordinary income and/or capital gains, unless you are invested through a tax-deferred arrangement, such as a 401(k) plan or an individual retirement account.

Payments to broker-dealers and other financial intermediaries

If you purchase shares of the Fund through an intermediary, the Fund and its distributor and/or the Adviser may pay the intermediary for services provided to the Fund and its shareholders. The Adviser and/or distributor may also pay the

intermediary for the sale of Fund shares. These payments may create a conflict of interest by influencing the intermediary and your salesperson to recommend the Fund over another investment. Ask your salesperson or visit your intermediary's website for more information.