Sunderland sacks manager O'Neill
The 61-year-old's departure was confirmed hours after Saturday afternoon's 1-0 home defeat by Manchester United, which left the club sitting just a point clear of the Barclays Premier League relegation zone with seven games left to play.
A club statement said: "Sunderland AFC has announced that it has parted company with manager Martin O'Neill this evening. The club would like to place on record its thanks to Martin and wishes him well for the future. An announcement will be made in the coming days regarding a successor."
A run of eight league games without a win which has yielded only three points ultimately cost O'Neill his job after a little more than 15 months at the helm.
The Black Cats' last victory came at Wigan on January 19, and with trips to European champions Chelsea and rivals Newcastle to come ahead of bogey team Everton's visit to the Stadium of Light on April 20, things could get worse before they get better.
O'Neill, of course, was the man whose appointment Sunderland fans had craved for years, and his arrival at the Stadium of Light in December 2011 was greeted with delight.
The Northern Irishman had never made any secret of his boyhood affection for the club and the chance to manage them fulfilled a long-held dream. His start was just what owner and chairman Ellis Short was looking for as a run of seven victories in his first 10 league games eased the club out of relegation trouble and sent spirits soaring.
The Black Cats ran out of steam towards the end of the campaign, but with their top-flight status secured and the promise of a summer rebuilding program, attention turned to the new campaign with genuine optimism.
That only increased when Short provided O'Neill with a total of £22million with which to buy Scotland international striker Steven Fletcher and England winger Adam Johnson, and the American put his hand in his pocket once again in January to add Danny Graham and Alfred N'Diaye to the squad.
However, only Fletcher, who was this week ruled out for the remainder of the season with ankle ligament damage, has hit the ground running with his 11 goals a major contribution to the 31 points they have claimed to date.
Johnson in particular has failed to recreate the form he showed in his early days at Middlesbrough and then in spells after his move to Manchester City, while Graham is yet to find the back of the net in seven appearances.