The 24-year old has been in stellar form for the Black Cats this season and O'Neill has backed him to go one step further and challenge Chelsea's Atletico Madrid loanee Thibaut Courtois to be Belgium's number one.
"He really wants to be good. His attitude is fantastic," O'Neill told reporters. "He is always striving to improve. Even on some days off, he wants to come in and do the work.
"I think that determination to be really good at his trade has shone through, it really has shone through.
"Of course, we all know there's a fair difference between having the determination to do it and then allying that with the ability, but he is improving all the time. He has been brilliant.
"He is in severe competition with the big Belgian lad as well to be the number one goalkeeper and with Belgium having such a fine side at the moment, that would be big for him."
Mignolet has kept seven clean sheets so far this campaign despite the fact Sunderland sit just four points above the relegation zone.