Keane spent three years as a Forest player before leaving to join Manchester United in 1993 for a then British-record transfer fee of £3.75m.

The 47-year-old worked alongside O'Neill during his five-year spell in charge of the Republic of Ireland, which ended in November.

O'Neill had said he would "dearly like" Keane to join him at the City Ground.

Keane told the club website: "Obviously, I had great memories of my career here as a player, the club gave me my chance and I will always remember them for that.

"We will give it a good go, I have huge respect for Martin and the club and it fell into place nicely.

"I have seen the team a number of times over the last year or two and it is going to be tough but hopefully, with one or two signings, a bit of luck we can hopefully be competing."

Forest are ninth in the Championship and four points off the play-offs, having won one and lost one of their two matches since O'Neill succeeded Aitor Karanka as manager.

Keane had spells as manager of Sunderland and Ipswich Town before being part of the Republic of Ireland set-up when they reached the last 16 of the 2016 European Championship.

As a player, he appeared in FA Cup and League Cup finals for Forest after joining from Irish club Cobh Ramblers in 1990.

He later won seven Premier League titles, four FA Cups and a Champions League during more than a decade with Manchester United.

Roy Keane made 156 appearances for Forest as a player, scoring 34 goals