PIMCO Corporate & Income Opportunity Fund (PTY) N-CSRSCertified semi-annual shareholder report (management investment company)

We believe that bond funds have an important role to play in a well-diversified investment portfolio. It is important to note, however, that in an environment where interest rates may trend upward, rising rates would negatively impact the performance of most bond funds, and fixed-income securities and other instruments held by a Fund are likely to decrease in value. A wide variety of factors can cause interest rates or yields of U.S. Treasury securities (or yields of other types of bonds) to rise (e.g., central bank monetary policies, inflation rates, general economic conditions, etc.). In addition, changes in interest rates can be sudden and unpredictable, and there is no guarantee that Fund management will anticipate such movement accurately. A Fund may lose money as a result of movements in interest rates.

As of the date of this report, interest rates in the United States and many parts of the world, including certain European countries, continue to increase. In efforts to combat inflation, the U.S. Federal Reserve raised interest rates multiple times in 2022 and 2023. Thus, bond funds currently face a heightened level of risk associated with rising interest rates and/or bond yields. This could be driven by a variety of factors, including but not limited to central bank monetary policies, changing inflation or real growth rates, general economic conditions, increasing bond issuances or reduced market demand for low yielding investments. Further, while bond markets have steadily grown over the past three decades, dealer inventories of corporate bonds are near historic lows in relation to market size. As a result, there has been a significant reduction in the ability of dealers to “make markets”.

Bond funds and individual bonds with a longer duration (a measure used to determine the sensitivity of a security’s price to changes in interest rates) tend to be more sensitive to changes in interest rates, usually making them more volatile than securities or funds with shorter durations. All of the factors mentioned above, individually or collectively, could lead to increased volatility and/or lower liquidity in the fixed income markets or negatively impact a Fund’s performance or cause a Fund to incur losses.

A Fund may enter into opposite sides of multiple interest rate swaps or other derivatives with respect to the same underlying reference instrument (e.g., a 10-year U.S. treasury) that have different effective dates with respect to interest accrual time periods also for the principal purpose of generating distributable gains (characterized as ordinary income for tax purposes) that are not part of a Fund’s duration or yield curve management strategies. In such a “paired swap transaction”, a 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

In February 2022, Russia launched an invasion of Ukraine. As a result, Russia and other countries, persons and entities that have provided material aid to Russia’s aggression against Ukraine, have been the subject of economic sanctions and import and export controls imposed by countries throughout the world, including the United States. Such measures have had and may continue to have an adverse effect on the Russian, Belarusian and other securities and economies, which may, in turn, negatively impact a Fund. The extent, duration and impact of Russia’s military action in Ukraine, related sanctions and retaliatory actions are difficult to ascertain, but could be significant and have severe adverse effects on the region, including significant adverse effects on the regional, European, and global economies and the markets for certain securities and commodities, such as oil and natural gas, as well as other sectors. Further, a Fund may have investments in securities and instruments that are economically tied to the region and may have been negatively impacted by the sanctions and counter-sanctions by Russia, including declines in value and reductions in liquidity. The sanctions may cause a Fund to sell portfolio holdings at a disadvantageous time or price or to continue to hold investments that a Fund may no longer seek to hold. PIMCO will continue to actively manage these positions in the best interests of a Fund and its shareholders.

The Funds may invest in certain instruments that rely in some fashion upon the London Interbank Offered Rate (“LIBOR”). LIBOR was traditionally an average interest rate, determined by the ICE Benchmark Administration, that banks charge one another for the use of short-term money. The United Kingdom’s Financial Conduct Authority, which regulates LIBOR, has announced plans to ultimately phase out the use of LIBOR. Although the transition process away from LIBOR for many instruments has been completed, some LIBOR use is continuing and there are potential effects related to the transition away from LIBOR or continued use of LIBOR on a Fund, or on certain instruments in which the Fund invests, which can be difficult to ascertain, and may vary depending on factors that include, but are not limited to: (i) existing fallback or termination provisions in individual contracts and (ii) whether, how, and when industry participants develop and adopt new reference rates and fallbacks for both legacy and new products and instruments. On March 15, 2022, the Adjustable Interest Rate (LIBOR) Act was signed into law. This law provides a statutory fallback mechanism on a nationwide basis to replace LIBOR with a benchmark rate that is selected by the Board of Governors of the Federal Reserve System based on the Secured Overnight Financing Rate (“SOFR”) for tough legacy contracts. On February 27, 2023, the Federal Reserve System’s final rule in connection with this law became effective, establishing benchmark replacements based on SOFR and Term SOFR (a forward-looking measurement of market expectations of SOFR implied from certain derivatives markets) for applicable tough legacy contracts governed by U.S. law. In addition, the FCA has announced

Issuers in which a Fund may invest can be affected by volatility in the banking sector. Even if banks used by issuers in which the Funds invest remain solvent, continued volatility in the banking sector could contribute to, cause or intensify an economic recession, increase the costs of capital and banking services or result in the issuers being unable to obtain or refinance indebtedness at all or on as favorable terms as could otherwise have been obtained. Conditions in the banking sector are evolving, and the scope of any potential impacts to the Funds and issuers, both from market conditions and also potential legislative or regulatory responses, are uncertain. Such conditions and responses, as well as a changing interest rate environment, can contribute to decreased market liquidity and erode the value of certain holdings, including those of U.S. and non-U.S. banks. Continued market volatility and uncertainty and/or a downturn in market and economic and financial conditions, as a result of developments in the banking sector or otherwise (including as a result of delayed access to cash or credit facilities), could have an adverse impact on the Funds and issuers in which they invest.

On each Fund Summary page in this Shareholder Report, the Average Annual Total Return table and Cumulative Returns chart measure 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’s distribution rate or that the rate will be sustainable in the future.

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 their complete schedules of portfolio holdings with the SEC for the first and third quarters of each fiscal year as an exhibit to their reports on Form N-PORT. The Funds’ Form N-PORT reports are available to the public on the SEC’s website at www.sec.gov and on PIMCO’s website at www.pimco.com, and upon request by calling PIMCO at (844)

SEC rules allow the Funds to fulfill their obligation to deliver shareholder reports 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 had phased compliance, with the latest requirement taking effect as of February 1, 2023.

In May 2022, the SEC proposed a framework that would require certain registered funds (such as the Funds) to disclose their environmental, social, and governance (“ESG”) investing practices. Among other things, the proposed requirements would mandate that funds meeting three pre-defined classifications (, integrated, ESG focused and/or 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.