O'Neill returned to Nottingham Forest, where he was a much-celebrated player, in January to take charge of the club from Aitor Karanka, who left the club in ninth in the Championship table.
Indeed, some have turned against O'Neill, after Keane left the club last week.
O'Neill and Keane were together for five years with Ireland and reformed their partnership when returning to a club they both played for.
O'Neill enjoyed a trophy laden spell at Forest during his playing days, winning the First Division title, two European Cups and one League Cup during his time at the City Ground under Brian Clough's guidance.
"The club would like to thank Martin for all his efforts during his time at The City Ground and wish him well for the future".
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Martin O'Neill is no longer the Nottingham Forest manager.
The ex-France global managed the Ivory Coast at the 2014 World Cup and more recently led Rennes to a fifth place finish in Ligue 1 in 2017/18.
Lamouchi brings plenty of experience to the role, having led Ivory Coast to the 2014 World Cup as the top-ranked African side and the 2013 African Cup of Nations quarter-finals.
Nottingham Forest is an opportunity for him to get back into the game and they will be aiming for the Play-Offs next season.
The former France global will be assisted by six new members of staff, the club confirmed.