The Northern Irish manager had been with Aston Villa for four years and had seen the club rise up the Premier League and delve in search for cup and European glory, not to mention finishing sixth in an increasingly difficult league three seasons in a row, each season gaining more points than the last.
Taking the club to Wembley twice last season, one final and one semi final, the first time Villa had been there in ten years is another notable achievement. There was a lot of speculation that O'Neill would leave Villa having taken them as far as they could go, but when the former Leicester boss signed on for another year the hearts of Villa fans where lifted as their saviour was still committed to taking them as high as he possibly could.
When the news dropped that he'd resigned, the shock factor is for the most part total in all circles. A few people have said that they are happy - the majority devastated. Speculation seems to be that O'Neill wanted the funds from the transfer of James Milner, but had been turned down.
O'Neill has shown himself to be an all or nothing manager, a man of principle. He will take all that is available to him and do the best he possibly can, or he will not do anything and simply not compromise. Yesterday's news certainly suggests that is the case.
With Villa due to start the new season against West Ham in four days, there are plenty of questions left to be answered. Will Milner still represent the Villains next season?
Will the new manager be on the way, and is he American?
What does Rand Lerner feel about the resignation, and what does this mean as far as Aston Villa and the transfer window are concerned?
There isn't much time for answers, so the quick decisions need to be the right ones.
Disclaimer: The views in this article are that of the writer and may not replicate those of the Professional Footballers' Association.