The 24-year-old Belgian has been in superb form for the Black Cats this season with his obstinacy at one end proving as valuable as striker Steven Fletcher's potency at the other.
Mignolet is involved in a fight for the number one spot for Belgium with 20-year-old Chelsea starlet Thibaut Courtois, who is currently on loan at Atletico Madrid, and he will be between the posts once again as Sunderland attempt to ease their way further clear of the Barclays Premier League relegation zone against West Ham.
O'Neill has seen at close hand his hunger to make it to the top and has been hugely impressed by the former Sint-Truiden keeper, who was has named the Player of the Year for 2012 by the north-east branch of the Football Writers' Association, he inherited from predecessor Steve Bruce.
He said: "He really wants to be good. His attitude is fantastic. 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 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.
"But he has been brilliant."