PIMCO Corporate & Income Opportunity Fund (PTY) N-CSR/ACertified annual shareholder report (management investment company) (amended)

Fund would generally enter into one or more interest rate swap agreements whereby a Fund agrees to make regular payments starting at the time the Fund enters into the agreements equal to a floating interest rate in return for payments equal to a fixed interest rate (the “initial leg”). A Fund would also enter into one or more interest rate swap agreements on the same underlying instrument, but take the opposite position (i.e., in this example, a Fund would make regular payments equal to a fixed interest rate in return for receiving payments equal to a floating interest rate) with respect to a contract whereby the payment obligations do not commence until a date following the commencement of the initial leg (the “forward leg”).

A Fund may engage in investment strategies, including those that employ the use of paired swaps transactions, the use of interest rate swaps to seek to capitalize on differences between short-term and long-term interest rates and other derivatives transactions, to, among other things, seek to generate current, distributable income, even if such strategies could potentially result in declines in the Fund’s net asset value (“NAV”). A Fund’s income and gain-generating strategies, including certain derivatives strategies, may generate current income and gains taxable as ordinary income sufficient to support monthly distributions even in situations when a Fund has experienced a decline in net assets due to, for example, adverse changes in the broad U.S. or non-U.S. equity markets or a Fund’s debt investments, or arising from its use of derivatives. For instance, a portion of a Fund’s monthly distributions may be sourced from paired swap transactions utilized to produce current distributable ordinary income for tax purposes on the initial leg, with a substantial possibility that a Fund will later realize a corresponding capital loss and potential decline in its NAV with respect to the forward leg (to the extent there are not corresponding offsetting capital gains being generated from other sources). Because some or all of these transactions may generate capital losses without corresponding offsetting capital gains, portions of a Fund’s distributions recognized as ordinary income for tax purposes (such as from paired swap transactions) may be economically similar to a taxable return of capital when considered together with such capital losses.

Classifications of the Funds’ portfolio holdings in this report are made according to financial reporting standards. The classification of a particular portfolio holding as shown in the Allocation Breakdown and Schedule of Investments sections of this report may differ from the classification used for the Funds’ compliance calculations, including those used in the Funds’ prospectus, investment objectives, regulatory, and other investment limitations and policies, which may be based on different asset class, sector or geographical classifications. Each Fund is separately monitored for compliance with respect to prospectus and regulatory requirements.

The common shares of the Funds trade on the New York Stock Exchange. As with any stock, the price of a Fund’s common shares will fluctuate with market conditions and other factors. If you sell your common shares of a Fund, the price received may be more or less than your original investment. Shares ofmanagement investment companies, such as the Funds, frequently trade at a discount from their NAV and may trade at a price that is less than the initial offering price and/or the NAV of such shares. Further, if a Fund’s shares trade at a price that is more than the initial offering price and/or the NAV of such shares, including at a substantial premium and/or for an extended period of time, there is no assurance that any such premium will be sustained for any period of time and will not decrease, or that the shares will not trade at a discount to NAV thereafter.

On each applicable Fund Summary page in this Shareholder Report, the Average Annual Total Return table measures performance assuming that any dividend and capital gain distributions were reinvested. Total return is calculated by determining the percentage change in NAV or market price (as applicable) in the specified period. Returns do not reflect the deduction of taxes that a shareholder would pay on Fund distributions. Total return for a period of more than one year represents the average annual total return. Performance at market price will differ from results at NAV. Although market price returns tend to reflect investment results over time, during shorter periods returns at market price can also be influenced by factors such as changing views about a Fund, market conditions, supply and demand for the Fund’s shares, or changes in the Fund’s dividends. Performance shown is net of fees and expenses. Historical NAV performance for a Fund may have been positively impacted by fee waivers or expense limitations in place during some or all of the periods shown, if applicable. Future performance (including total return or yield) and distributions may be negatively impacted by the expiration or reduction of any such fee waivers or expense limitations.

The dividend rate that a Fund pays on its common shares may vary as portfolio and market conditions change, and will depend on a number of factors, including without limit the amount of a Fund’s undistributed net investment income and net short- and long-term capital gains, as well as the costs of any leverage obtained by a Fund. As portfolio and market conditions change, the rate of distributions on the common shares and a Fund’s dividend policy could change. There can be no assurance that a change in market conditions or other factors will not result in a change in a Fund distribution rate or that the rate will be sustainable in the future.

by the Funds as the policies and procedures that PIMCO will use when voting proxies on behalf of the Funds. A description of the policies and procedures that PIMCO uses to vote proxies relating to portfolio securities of each Fund, and information about how each Fund voted proxies relating to portfolio securities held during the most recent twelve-month period ended June 30th, are available without charge, upon request, by calling the Funds at (844)on the Funds’ website at www.pimco.com, and on the Securities and Exchange Commission’s (“SEC”) website at www.sec.gov.

The Funds file portfolio holdings information with the SEC onwithin 60 days of the end of each fiscal quarter. The Funds’ complete schedules of securities holdings as of the end of each fiscal quarter will be made available to the public on the SEC’s website at www.sec.gov and on PIMCO’s website at www.pimco.com, and will be made available, upon request, by calling PIMCO at (844)

SEC rules allow shareholder reports to be delivered to investors by providing access to such reports online free of charge and by mailing a notice that the report is electronically available. Investors may elect to receive all reports in paper free of charge by contacting their financial intermediary or, if invested directly with a Fund, investors can inform the Fund by calling (844) 33-PIMCO. Any election to receive reports in paper will apply to all funds held with the fund complex if invested directly with a Fund or to all funds held in the investor’s account if invested through a financial intermediary, such as a broker-dealer or bank.

In April 2020, the SEC adopted amended rules modifying the registration, communications, and offering processes for registeredfunds and interval funds. Among other things, the amendments: (1) permit qualifyingfunds to use a short-form registration statement to offer securities in eligible transactions and certain funds to qualify as Well Known Seasoned Issuers; (2) permit interval funds to pay registration fees based on net issuance of shares in a manner similar to mutual funds; (3) requirefunds and interval funds to include additional disclosures in their annual reports; and (4) require certain information to be filed in interactive data format. The new rules have phased compliance dates, with some requirements having already taken effect and others requiring compliance as late as February 1, 2023.

In October 2020, the SEC adopted a rule related to the use of derivatives, short sales, reverse repurchase agreements and certain other transactions by registered investment companies that rescinds and withdraws the guidance of the SEC and its staff regarding asset segregation and cover transactions that was applicable to the Funds as of the date of this report. Subject to certain exceptions, the rule requires funds that trade derivatives and other transactions that create future payment or delivery obligations to comply with a value-at-risk

impact funds) provide prospectus and shareholder report disclosure related to the ESG factors, criteria and processes used in managing the fund. The proposal’s impact on the Funds will not be known unless and until any final rulemaking is adopted.

The following example illustrates the expenses that you would pay on a $1,000 investment in Common Shares of the Fund, assuming (1) that the Fund’s net assets do not increase or decrease, (2) that the Fund incurs total annual expenses of 1.21% of net assets attributable to Common Shares in years 1 through 10 (assuming assets attributable to Preferred Shares and reverse repurchase agreements representing 34.48% of the Fund’s total managed assets) and (3) a 5% annual return

The following example illustrates the expenses that you would pay on a $1,000 investment in Common Shares of the Fund, assuming (1) that the Fund’s net assets do not increase or decrease, (2) that the Fund incurs total annual expenses of 1.87% of net assets attributable to Common Shares in years 1 through 10 (assuming assets attributable to Preferred Shares and reverse repurchase agreements representing 34.26% of the Fund’s total managed assets) and (3) a 5% annual return