Sacked apparently -
"He can count himself unlucky to have lost his job with the team just one point away from the top six," Forest chairman Fawaz Al Hasawi said.
"With the January transfer window approaching, we feel it's the right time to make a change."
Al Hasawi added that Forest would now be "looking to bring in an ambitious manager with Premier League experience".
Former Republic of Ireland international O'Driscoll was appointed on 19 July, days after Steve Cotterill was sacked.
He leaves Forest with them in eighth place, having collected 36 points from 24 games. In total, Forest won 10 games during his time in charge, drawing nine and losing seven.
However, they had not won more than two games in succession under O'Driscoll, and Al Hasawi cited that lack of consistency as one of the reasons for the decision.
"We have a responsibility to look to the future for this great club because we have huge ambitions for it," he added.
"We knew when we bought the club in the summer that it would take time for the players we bought in to settle but that process has taken longer than we anticipated.
"We feel we have developed a really strong squad of players but are still searching for consistency in terms of team performance, underlined by the fact that we have not won more than two games in succession in the Championship this season."
O'Driscoll spent five months as first-team coach at Forest at the start of the year, before leaving to take charge of Crawley in May.
He was with the League One side for just two months before returning to the City Ground as manager, signing a three-year contract.
O'Driscoll's task was to build a side capable of challenging for promotion to the Premier League and he made a number of high-profile signings, including Simon Cox from West Bromwich Albion, Henri Lansbury from Arsenal and Billy Sharp on loan from Southampton.
Shortly after the news of O'Driscoll's sacking was confirmed, Cox wrote on Twitter: "So we win 4-2 and the manager gets the sack. Gutted for Sean, pleasure to work under. Wonder who will walk through the door next."
Sharp also took to Twitter to give his reaction, writing: "Gutted that ruins a decent day."
http://www.bbc.co.uk/sport/0/football/20847476