Authorized congressional committees, appropriate Inspectors General, designated agency officials, or the Office of Special Counsel.
OSC can accept classified information, the disclosure of which, if made publicly, would be “specifically prohibited by law.”
5 U.S.C. 1213, which governs OSC’s procedures for accepting disclosures, works as follows:
Section (a)(1) applies to disclosures made anywhere, provided that the disclosure is not specifically prohibited by law.
Section (a)(2) applies to disclosures made to the IG, designated agency official, or OSC, without explicit prohibition as to type of disclosure.
Under the statutory canon of expresio unios, the presence of the “specifically prohibited by law” qualifier in (a)(1) but not in (a)(2) implies that certain disclosures are lawful when made to OSC, but unlawful when made in public.
Further, section (j) states that, with regard to disclosures involving “foreign intelligence or counterintelligence information,” OSC “shall transmit such information to the National Security Advisor, the Permanent Select Committee on Intelligence of the House of Representatives, and the Select Committee on Intelligence of the Senate,” instead of abide by the usual agency/Congress/President/public mechanism for non-classified disclosures.
Finally, the legislative history of the Whistleblower Protection Act of 1989 (page 26) supports this view:
and page 28: