The 19-year old striker has failed to live up to early expectations since arriving at the Stadium of Light from Ipswich Town in June last year and O'Neill has urged to be a more aggressive, attacking player in order to produce more performances like his second-half showing against Norwich City on the weekend.
"Connor asked me what things he needed to address and aggression is very important," the Black Cats boss told the club's official website.
"That doesn't mean running around, kicking players and slamming into people - and Connor didn't take it in that manner.
"It means being aggressive in his own game - it means when the ball is there to be headed, going higher than anyone else and heading it full force.
"Winning flick-ons is totally different to full-blooded headers in the opponents' penalty area. Those are the kinds of things I expect."