The England winger joined the Black Cats from Manchester City in the summer but has yet to fire on Wearside mainly due to injuries.
O'Neill is well aware of Johnson's ability but is remaining patient in regards to his output so early in his Sunderland career.
"I'm just a little concerned that people might be expecting him to come in and be winning every game for us because that's certainly not going to be the case," he told the Sunderland Echo.
"He's a real talent, and I know he's going to be a great asset to this club. But he's only one player and it's a team game.
"Creatively, we have to be better and he has a part to play in that, but then so do other players.
"It's about everyone taking responsibility and working hard to make things happen."