The pair were axed from the Republic of Ireland set-up after a run of disastrous results, but the duo will now return to management and a club that they both played for.
Forest announced that their former player, who kicked off his career there, was returning to the club as assistant manager.
Almost thirty years after signing for the club, under Brian Clough, the former midfielder is back at the City Ground in a bid to help the club into the Championship play-off places.
Keane and O'Neill forged a great management team and guided the national side to Euro 2016, but quit in November after a dismal run.
The two former Forest players have their work cut out, as the Reds currently sit in ninth; four points and three positions off Derby.
Keane, 47, was presented with an offer from O'Neill who took over from Aitor Karanka a matter of weeks ago.
The former Manchester United ace was weighing up the proposition, deciding whether to wait for his own job offer as top dog, or to take on another No.2 role.
After seeing the ambitious plans by Forest's owner Evangelos Marinakis, Keane agreed to the role in a bid to end the club's 20 year exile from the Premier League.
O'Neill made it clear, when he was announced as the club's new boss, that he wanted to reunited with Keane.
He said: "I would dearly like him to join us.
"Overall, he was great for me, and great for the Ireland players, and the genuine truth is that I don't think that we would have qualified for the Euros without his presence.
"I have been unflinching in that support."
O'Neill stood by his colleague when players slammed Keane's direct approach and fell out with some of the players.
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