O'Neil became Bournemouth's permanent manager on a one-and-a-half year contract in November 2022, with the option to extended by 12 monthsBournemouth have sacked Gary O'Neil less than seven months after he was appointed as the club's permanent manager.
The 40-year-old took charge as interim boss after Scott Parker was sacked in August, then was appointed on a permanent basis in November.
O'Neil won 10 of his 34 Premier League games in charge last season, guiding Bournemouth to a 15th-placed finish.
"This has been a difficult decision," Bournemouth owner Bill Foley said.
"It has been made with great consideration to best position ourselves ahead of the coming season.
"Gary will go on to have a long career as a head coach or manager, but we feel that, at this moment in time, a change is in the best interests of this football club."
Bournemouth have said they will announce O'Neil's successor "imminently".
O'Neill took the helm at Vitality Stadium after Parker was sacked following a 9-0 hammering by Liverpool - the joint biggest defeat in Premier League history.
It was the club's third successive defeat, but O'Neil went unbeaten in his first six games in charge.
He earned 13 points from 11 top-flight games in interim charge and signed a one-and-a-half year contract with the option of a 12-month extension when he was made the club's permanent manager during the Premier League's mid-season break for the World Cup.
That was before a partnership led by American businessman Foley completed its takeover of Bournemouth in December.
It was O'Neil's first managerial position after assistant coaching roles with Liverpool Under-23s and with Bournemouth from February 2021 when the club was in the Championship.
In the 23 Premier League games following the World Cup break, O'Neil's Bournemouth lost 14 and won seven, including a 1-0 victory over Liverpool in March and a 3-2 away win against Tottenham in April.
Foley said Bournemouth had identified "a number of significant targets in the transfer market" and O'Neil's departure would provide the club with "the best platform from which to build".
Former Bournemouth player and manager John Williams told BBC Radio Solent he was "totally shocked" by the decision.
"I thought the Cherries were moving along nicely and building for the pre-season coming up," Williams said.
"The first thing that springs to mind is, they must have someone lined up.
"Without a doubt Gary O'Neill will feel hard done by. I doubt he will be out of work for long."
