McClean started the season at Derry City, but hasn't looked back since breaking into the Sunderland first team.
O'Neill now believes McClean is the player the opposition teams will target.
"James McClean is a marked man now." O'Neill told safc.com.
"What we have to try and do is to devise ways in which he can receive the ball, where he gets a little help from us.
"Back in his early stages he was an unknown quantity and he was able to take on players one versus one, but they're backing up now and we have to find ways of helping him."
"If James has two players on him, then there has to be someone free for us and we have to try and work that one and help James out."