The 66-year-old, who played 371 games and won the European Cup as a Forest player, ended his five-year spell as Republic of Ireland boss last November.

He returns to a Reds side who are ninth in the Championship, four points outside the play-off places.

O'Neill will become Nottingham Forest's 11th manager since June 2011.

Karanka's exit from the City Ground - with Forest still very much in the promotion race - means O'Neill will arrive in a strong position both on the field and off it.

After finishing 17th in 2017-18, they invested heavily during the summer, with Joao Carvalho arriving from Benfica for a club record fee of £13.2m and striker Lewis Grabban joining in a £6m deal from Bournemouth.

The Forest job would be the Northern Irishman's first return to club management since a mixed spell with Sunderland, which saw him guide the Black Cats away from relegation from the Premier League in his first season but then struggle again the following campaign prior to his sacking in March 2013.

Will O'Neill match his Forest playing legacy?

Voted into Forest's all-time greatest XI in 2016, O'Neill has considerable stock with supporters.

He was part of the Brian Clough side that rose from the second division to champions of Europe, playing in the 1980 European Cup final having missed out on a place in the team 12 months earlier when goalscorer Trevor Francis took his place.

As a manager, O'Neill enjoyed a meteoric beginning to his career.

He guided Wycombe Wanderers into the English Football League, twice won the League Cup with Leicester City and led Celtic to three Scottish Premiership titles, three Scottish Cups and a League Cup - as well as the 2003 Uefa Cup final.

However, his experiences since coaching Aston Villa, who he guided to three consecutive sixth-place finishes in the Premier League, have failed to produce the same success.

Clark, Pearce... now O'Neill?

O'Neill is not the first club favourite to return to Forest in a managerial capacity, with Frank Clark and Stuart Pearce both achieving mixed results.

Clark, who was named as Brian Clough's successor in 1993 at a time when then-Wycombe boss O'Neill was linked to the post, led Forest to promotion and into Europe with a third-placed finish in the Premier League.

However, Forest struggled following the departure of striker Stan Collymore to Liverpool and, amid off-field ownership uncertainty, he eventually left in December 1996.

Pearce's first taste of management was as caretaker in the aftermath of Clark's exit - and resulted in a manager of the month award before he relinquished the role when Dave Bassett was appointed.

He returned as full-time manager to huge emotional fanfare in July 2014.

The highlights - including four wins and a draw from the opening five games and victory against rivals Derby County in one of his final matches in charge - were offset by a run of just three wins in 23 games and he departed with Forest in 12th place in the Championship in early 2015.