Keane was O'Neill's assistant on two occasions, between 2013 and 2018 for the Republic of Ireland before the pair joined forces again as No 1 and No 2 at Nottingham Forest in 2019.
"He's got the personality for a start," O'Neill - who was also a highly successful manager at Celtic - told BBC Scotland Sportsound.
"He's very strong-willed, and I know people then start to feel strong-willed people do not necessarily change or adapt. He has certainly got the capacity to do that - to motivate players, absolutely.
"I worked with him at close hand. Sometimes in this game you get a reputation and that supersedes anything that you're trying to do. Roy Keane has learned a great deal in the intervening years.
"He was my assistant at Ireland and I know that's a totally different job, but has Roy Keane got the personality to manage at the very top? Absolutely."
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