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 tomorrow 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 today 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."
Mignolet's reliability is a major plus for O'Neill as he attempts to piece together a side from a squad which is being increasingly stretched by injury.
Defenders Danny Rose and Carlos Cuellar will be sidelined for some time by the hamstring complaints they picked up during last Saturday's 2-2 FA Cup third round draw at Bolton, while John O'Shea, who has a similar problem, has returned to training but remains a doubt.
As a result, midfielders Craig Gardner and Jack Colback could once again find themselves pressed into service as emergency full-backs.
O'Neill said: "It's not ideal for us, really, and that's a problem we have had to face for most of the season.
"But we have to cope with it, that's the way it is."
Playmaker Stephane Sessegnon is also back in training after an ankle injury and O'Neill will also hope to have £4million signing Alfred N'Diaye available for the first time against the Hammers.
The club is still awaiting international clearance for the 22-year-old French midfielder, but has until an hour before kick-off for the documentation to come through.
O'Neill has been keen to add physicality to his midfield resources and with N'Diaye standing at in excess of 6'2", he may have done just that.
He said: "He is athletic. Of course he is young enough that he can improve his own game and he's got a lot of determination about him.
"I think in time he will definitely improve us. He will give us a bit of extra vitality.
"But he hasn't played in the Premier League before, so it is going to be something new to him, but he is really up for the challenge."
O'Neill's efforts to strengthen his squad are ongoing, although he was giving little away on Friday.
Swansea striker Danny Graham is a target, although Sunderland face competition, while Senegal international defender Kader Mangane, currently on the books at Saudi Arabian club Al Hilal, is understood to be due on Wearside over the weekend.
In the short term, however, O'Neill's focus will be on the three points at stake tomorrow and the task of ensuring they do not leave the north-east with Sam Allardyce's men.
He said: "It might be surprising to some people, but not really to me. You know what to expect from Sam's teams, and they have done exceptionally well this season, exceptionally well."