Aston Villa manager states that the striker's goal should have stood, and says that the officials need to start getting decisions right after some high profile incidents
The match itself was a very close, leaving O'Neill criticising the linesmen and Alex McLeish disappointed with a number of missed chances.
Danish striker Bendtner tapped in at the far post following a low cross from Seb Larsson on the first hand side, but it was chalked off with it being claimed the on-loan Arsenal man was offside, and O’Neill wasn't impressed.
"We had a lot of chances and I’m disappointed not to take any of them. I think the linesman has incorrectly called the offside because he [Bendtner] was level," O'Neill told reporters.
"It was on [onside] certainly anyway. Unless they changed the rules this morning, then it was still on.
"They’re having a hard time at the minute, linesmen and referees. They’ve got to try and get that right, particularly when I think he told one of our players to watch it on TV, which was very magnanimous of him. He [the linesman] was so adamant he got it right.
"It's nice that confidence, self confidence that some people have," he said sarcastically.
The Northern Irishman also spoke of his reception on his first Premier League return to Villa Park since he walked out on the club just before the start of last season, but his frosty welcoming was nothing less than he expected.
He continued: "I think it was a natural reaction. I was there for two years and they don’t know my reasons for leaving.
"The idea that I left them in the lurch a little, I apologise for, leaving five days before the start of the season.
"I came to [Aston Villa] a fortnight before the season started, a team that had almost been relegated the year before.
"I think the team was infinitely better than the team I had. Internationals galore, players that have gone on to other clubs and who have been excellent for the other clubs: [James] Milner, Ashley Young, [Stewart] Downing.
"In the scheme of things it’s not that important, it’s just a sideshow of a game between two teams."
As pointed out to the Sunderland boss, there were a few Villa fans applauding their former manager, but O’Neill quickly spoke of a different theory.
"I think the clapping may have been down to me putting some people in the crowd," he joked.
McLeish wasn't too down heartened either, but admitted he was disappointed they couldn't capitalise on any of their chances.
"I thought we started really well," he said. "We got the fans right behind us. They were terrific from start to finish.
"[Sunderland] are a very dangerous side with a lot of experience, but I feel we had created enough to win it.
"We still have four games to go and we have to try and take points in every single one and I think we are capable of doing that.
McLeish and Villa took two significant blows with the injuries of James Collins and Gabby Agbonlahor, and in terms of recovery time early indications are mixed.
"James Collins has a groin injury. We think he won’t make Tuesday but we’ll get the diagnosis tomorrow and Gabby’s hurt his shoulder but he does have a chance. I'm pleased there is a wee chance he will be okay.
McLeish was thankful for a stroke of luck in the second-half when Bendtner's goal was disallowed, but he insisted they were due a rub of the green.
"Yeah I saw it. His left foot was offside," said the former Aberdeen boss, jokingly. "Terrific spot by the linesman.
"We've had our fair share of decision go against us in recent weeks, but I don’t want to sit here and make excuses.”