The England Under-21 international has struggled to make an impact since arriving from Ipswich Town two summers ago, but looked lively after coming off the bench at half-time in Sunday's defeat against Norwich.
And O'Neill believes that the frontman would benefit from imposing himself more forcefully on games, particularly in front of goal.
“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.”
At well over six foot and with a muscular frame, Wickham should have no problem dealing with the physical side of the Premier League, and O'Neill believes that this gives the 19-year-old an excellent platform to build on.
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“He seems to have got stronger during my time here and now he doesn’t have to worry about being in the gym and building himself up.
“He can be very strong indeed and not being shoved around is very important in today’s game.
“When you feel you have that strength, the other aspects of a player’s game coming into being and more to the fore.
“You’re more comfortable with those aspects simply because you have that strength behind you.”