The former Leicester City and Aston Villa boss took over at the Stadium of Light in December, and has since guided the club to 10 wins in 17 during his first three months at the club.
One player who has flourished under the Northern Irishman's control in the north-east has been McClean.
But O'Neill admits he knew little about the Derry-born 22-year-old, who has since become a regular fixture in his side.
"I didn’t know him at all, I met him during my first week," O'Neill revealed to Yahoo!
"He had been substitute for a number of games without ever getting on the field of play, but I did see him play for our reserves against Manchester United during my first week at the club.
"I put him on against Blackburn in effectively my first game in charge of the team in the 75th minute when it looked like we had run out of ideas to break Blackburn down.
"He came on and gave us a boost, reignited us again, went past the full-back put in a cross and suddenly we got another lift. Since making his full debut he has never looked back."
McClean will be hoping to be handed a start for Sunderland in their upcoming FA Cup quarter-final tie with Everton, and O'Neill claims the competition is crucial to the club's current campaign.
"It has become important to us," he said. "We set out against Peterborough in January and managed to get through that round and each round since has been difficult but we are in the quarter-final now."
"Everton will be very formidable opposition and think that will be very very tough for us.
"Only eight teams are left so we should give it everything we have got, just as we have done in all the league games."
The former Nottingham Forest midfielder conceded that the FA Cup has lost a little of its sparkle in recent years, as European endeavours have taken centre stage for the bigger Premier League clubs.
"I think it has lost a bit of its sheen and that is probably because the Champions League has now become like the Holy Grail," said the 60-year-old.
"The Barclays Premier League has become very important and back when I was growing up there was only one live game of football during the year and that was the FA Cup final, so everybody tuned into it.
"This year it is not going to be played on the final day of the season, there will be a league game to negotiate after the final.
"It's a great competition nevertheless and I am hoping that it will regain not only its popularity but also its sheen in the coming years."
For more insight from Martin O’Neill and other leading managers, plus exclusive Barclays Premier League highlights, visitwww.yahoo.co.uk/sport
