Wolves remain 19th in the Premier League table, ahead of Southampton by four points, though the South Coast team have a game in hand.
Sourced have told ESPN that the club are in talks with Portuguese coach Vitor Pereira over replacing O'Neil. Pereira is in charge of Al Shabab in the Saudi Pro League and has previously coached at Porto and Fenerbahce, among other teams.
O'Neil signed a four-year extension in August after guiding the club to a 14th-place finish with 46 points in the 2023-24 campaign. However, his side have struggled defensively since that extension, conceding a league-worst 40 goals after 16 games.
"We're very grateful to Gary for all of his effort, dedication and hard work during his time at the club, and we wish him and his team the best of luck for the future." Wolves chairman Jeff Shi said in a club statement.
O'Neil, a former West Ham defender, becomes the third Premier League head coach to be relieved of his duties this season after Manchester United sacked Erik ten Hag and Leicester City fired Steve Cooper.
Wolves' next game is away at Leicester on Dec. 22, with tough fixtures coming up against Man United and Tottenham later in the month.
Information from ESPN's Rob Dawson contributed to this report.