Just a few years ago O'Neill was topping polls to be the next Manchester United manager and being linked with virtually every top job in Britain.
After achieving great things with minnows Wycombe Wanderers, a brief stint at Norwich City was followed by the job at Leicester City, where he made his name in a big way.
Two league cup wins in 1997 and 2000 and top ten finishes in the Premier League had Leicester punching way above their weight and made O'Neill a wanted man.
O'Neill also gained a reputation as a man of principle as he stayed at Leicester until his contract expired.
He then got his big chance with Celtic in Scotland and didn't disappoint. Despite a lack of opposition making things easier, he won a domestic treble and even brought success in Europe, making it to the Uefa Cup final, only losing in extra time to Jose Mourinho's Porto.
After winning three league titles, three FA Cups and a League Cup in a five-year stint O'Neill decided to take a break from management to look after his ill wife.
He returned with Aston Villa and took them to sixth place three years running as well as their first major cup final in 10 years in the League Cup losing to Manchester United.
O'Neill quit Villa in August 2010, reportedly due to lack of financial backing from chairman, Randy Lerner.
He then took over Sunderland in December 2011 but that has ended in tears with his first ever sacking.
So where has it all gone wrong for the man from Northern Ireland?
Many people felt the Sunderland job was possibly a bridge too far, saying that previous manager; Steve Bruce had left him with a squad of bad players.
The fact that his regular number two and former team mate John Robertson had for the first time refused to come with him to a club, choosing to retire instead, is another factor some have cited as an issue. O'Neill's hero and mentor, Brian Clough had famously always relied on his number two, Peter Taylor.
O'Neill was a boyhood Sunderland fan so he was always going to take the job, but 16 months in the job is hardly enough time to put his stamp on a squad.
Steven Fletcher's injury meaning he's out for rest of the season has also come at the worst time possible for the team.
O'Neill spoke of carrying on and a steely resolute in the dressing room after last weekend's defeat to Manchester United but little did he know this would be his last game in charge.
You have to feel sorry for O'Neill who seems a genuine and honest man of principle in the shady world of football, but it looks now if the really big jobs will elude him and can only hope that his next chairman gives him more time.
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