O'Neill moved to the Stadium of Light in December 2011 when the club were 16th in the table, and pushed them up into 13th spot by the end of the campaign.
"With the experience I've had, when I came into the club and it was pretty much on its knees and I think I saved the club from relegation, I think I should've been given more of an opportunity," O'Neill told BBC Radio Five Live.
"[I'm] pretty disappointed and frustrated as much as anything else. I'm in a business now where I think very little shocks you about professional football. I think you can lose your job if your tie doesn't fit your suit."
O'Neill was replaced by recently-sacked Swindon Town manager Paolo Di Canio, whose arrival has been marred by allegations that the Italian holds fascist beliefs.