O'Neil succeeds Rosenior as Strasbourg coach
Former Bournemouth and Wolves boss O'Neil heads to France after Rosenior left the modest Ligue 1 outfit on Tuesday for Premier League side Chelsea.
The 42-year-old had a playing career including spells at Portsmouth and Middlesbrough and represented England under-21s.
"I'm excited to get started and proud to be part of this fantastic club," O'Neil said in a Strasbourg statement.
"We have an exciting group of players and clear ambitious objectives for the season.
"My focus is on working hard with the squad to give everything for the club's success," he added.
O'Neil takes over with the Alsatians, who won their sole French title in 1979, in seventh place in Ligue 1 but without a win in the league since November 9.
They are top of the Conference League table, having beaten the likes of Crystal Palace and Aberdeen.
During his time in the Premier League O'Neil guided Bournemouth to top-flight safety and took over at Wolves after Julen Lopetegui's chaotic departure.
He left Molineux in December 2024 with the club in 19th place, having won just twice that season.
O'Neil's first game in charge of Strasbourg will be Saturday's French Cup last 32 game at fourth-tier Avranches.
'Humiliating step'
"I am very happy to welcome Gary O'Neil to Racing," Strasbourg president Marc Keller said.
"He is a demanding and recognised coach, with a modern approach to football that is fully part of the continuity of our sports project," he added.
Rosenior's move to Chelsea has been shrouded in controversy with the Blues and Strasbourg owned by the same consortium, BlueCo, who bought the three-time French Cup winners in June, 2023.
Strasbourg fans called for Keller to leave his role due to Rosenior's departure which followed the equally unpopular announcement club captain Emmanuel Emegha would head to Stamford Bridge next season.
Rosenior guided Strasbourg to seventh in the table last term, a marked improvement on his predecessor Patrick Vieira under whom they had finished 13th.
Strasbourg's last piece of silverware was lifting the fourth of their League Cup trophies in 2019.
"Liam Rosenior's move marks yet another humiliating step in Racing's subservience to Chelsea," the Strasbourg Supporters' Federation said on Facebook.
"The problem goes far beyond the mid-season sporting impact and the ambitions of a young coach. It is structural, it is the future of French club football that is at stake.
"Every additional contortion by Marc Keller, every extra minute spent at the helm of the club, is an insult to the tremendous work accomplished before 2023," it added.