Johnson was brought in for around £10million from Manchester City over the summer and the England man is seen as the wide man who can provide plenty of goals for the club this season.
He has made a positive start - although his progress has been hampered slightly by injury - and O'Neill, while aware how good the 25-year-old can become, says others must also step up to the plate in the final third.
The Black Cats boss said: "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'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."
Sunderland have made a solid start to the season, drawing four and losing once but only winning one of their six games, and O'Neill is satisfied although he wants more from his attacking and creative players.
He added: "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.
"We've made a solid start to the season and I'm not unhappy with it. Defensively we've been solid but we need to be more effective in the final third and that's what we're all working on.
"It's still the start of the season though and I know the squad will continue to improve as it progresses."