The Black Cats progress to round four courtesy of two fantastic second-half goals from Craig Gardner and James McClean but it was the dismissal of Cattermole that left O'Neill with a bitter taste in his mouth.
O'Neill felt his skipper gave referee Stuart Attwell little choice but to send him off for a first-half lunge on young MK Dons full-back Adam Chicksen. He said: "From the minute you start lunging in you are giving the referee no option. It looked from the sideline that the referee had no option."
The Sunderland manager added: "I think that whatever we say, we can watch it back a hundred times, the referee is watching someone lunge in and you run that risk, it is crazy.''
O'Neill, who has won the League Cup on two occasions as a manager with Leicester, felt Cattermole set a bad example for his team-mates. "You can't do it, we talked about discipline before the game and there we are, down to 10 men, it is a hard enough battle here and he didn't help himself and he didn't help his team,'' he said.
Gardner's tremendous 25-yard free-kick and an individual effort from McClean secured a relatively safe passage into the next round and O'Neill felt his players overcame a potentially tricky fixture to get there.
He said: "I don't want to go overboard about it but it is good to win. I was very concerned tonight considering some of the players we had to leave out of the team. To win tonight was great, considering the circumstances of playing most of the game with 10 men."
MK Dons manager Karl Robinson felt his side could have come out on top but was pleased with the way his players took the game to their top-flight opponents.
He said: "I thought we deserved a little bit more out of the game to be honest and I think most people in the ground would agree with that. What a wonder goal that was at 0-0 and that took the stuffing out of us.
"We had to go on the front foot, because we have got the man advantage and we were also a goal behind."
Source: PA
Source: PA