Company: PTY
Filing Date: 2025-03-03
Form Type: POS EX
Source: 0001193125-25-042988
Chunk: 1

Company: PIMCO CORPORATE & INCOME OPPORTUNITY FUND
Filing Date: 2025-03-03
Form: POS EX
Chunk 1
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 Securities Exchange Act of 1934 (“Exchange Act”).                                                                                                                                                   |
| ☐Interval Fund (RegisteredClosed-EndFund or a Business Development Company that makes periodic repurchase offers under Rule23c-3under the Investment Company Act).                                                                                                      |     | ☐If an Emerging Growth Company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of Securities Act. |
| ☒A.2 Qualified (qualified to register securities pursuant to General Instruction A.2 of this Form).                                                                                                                                                                     |     | ☐New Registrant (registered or regulated under the Investment Company Act for less than 12 calendar months preceding this filing).                                                                                                                             |

EXPLANATORY NOTE This Post-Effective Amendment No. 1 to the Registration Statement on Form N-2(File Nos. 333-274752and 811-21238)of PIMCO Corporate & Income Opportunity Fund (the “Registration Statement”) is being filed pursuant to Rule 462(d) under the Securities Act of 1933, as amended (the “Securities Act”), solely for the purpose of filing exhibits to the Registration Statement. Accordingly, this Post-Effective Amendment No. 1 consists only of a facing page, this explanatory note and Part C of the Registration Statement setting forth the exhibits to the Registration Statement. This Post-Effective Amendment No. 1 does not modify any other part of the Registration Statement. The contents of the Registration Statement are hereby incorporated by reference.

### PART C - OTHER INFORMATION

### Item 25: Financial Statements and Exhibits
**Included in Part A:

Financial highlights for: the fiscal year ended June 30, 2023, the fiscal period ended June 30, 2022, the fiscal years ended July 31, 2021, July 31, 2020 and July 31, 2019, the fiscal periods ended July 31, 2018, July 31, 2017, July 31, 2016 and July 31, 2015, and the fiscal years ended November 30, 2014, November 30, 2013 and November 31, 2012

Incorporated into Part B by reference to Registrant’s most recentCertified Shareholder Report on Form N-CSR/A, filed September 23, 2024 (File No. 811-21238