The 61-year-old's tenure at the Stadium of Light lasted 15 months before a 1-0 home defeat to league leaders Manchester United last Saturday left the Black Cats just a point off the relegation zone and O'Neill was dismissed later that evening.
|PREMIER LEAGUE LATEST
|2/5||Sunderland are 2/5 to avoid relegation from the Premier League with bet365
"I am still pretty disappointed, frustrated as much as anything else but life goes on," O'Neill told BBC Radio 5 Live.
"I'm in the business now where I think very little shocks you about professional football especially in the last 10 years. You can lose a job in management if your tie doesn't fit your suit.
"With the experience I have had over the years ... also coming into the football club at a time when the club was pretty well on its knees. I believed I saved it from relegation last year and I thought that opportunity should still have been afforded to me."
Sunderland lie one place and point outside the relegation zone with eight games remaining and face difficult fixtures against the likes of Champions League-chasing Chelsea and Tottenham and relegation rivals Aston Villa, Southampton and bitter rivals Newcastle.