Johnson has struggled to make an meaningful impact for O'Neill's side this season since his £10m move from Manchester City last August.
O'Neill told reporters: "Adam has to show consistency here. It might be simplifying things but he played cameo roles at Manchester City in an extraordinarily good team that won the league last season with brilliant players.
"People thought he was coming back to Sunderland, where he's from and that he was going to win games single-handedly and we were going to go from being a struggling side to one challenging for European honours."
The England international has made 31 appearances for Sunderland this season, scoring four goals.