The watchdog office says Comey broke FBI rules by giving a memo containing unclassified information to a friend with instructions to share the contents with a reporter.
The office also says Comey failed to notify the FBI after he was fired that he had retained some of the memos in a safe at home.
Comey wrote a series of memos about conversations with Trump he says unnerved him. He says he wanted to make a record of the interactions because he was concerned Trump might later lie about them.
The Justice Department has already decided that it won’t prosecute Comey over the matter.