The 25-year old has provided just four goals and four assists this term despite featuring regularly for the Black Cats since his €11.5 million arrival.
However, O'Neill has dismissed criticisms of the player, claiming that Johnson is still adapting to life at the club.
"When he was playing for Manchester City and was coming on with 20 minutes to go, it was a very different job to what he has now," the Northern Irishman told reporters.
"He has come to Sunderland and is almost expected to win games single-handedly - that is not my view, but might be held by a few supporters.
"There is a view that he should go past a couple of players and create something every time he gets the ball. It is a change and it is hard, and he has to come to terms with all of that."
Johnson has seemingly fallen out of favour with England manager Roy Hodgson, but O'Neill has backed him to win an international recall in the near future.
"I would be first to admit there is plenty left for him to produce," the former Aston Villa manager said.
"This is the type of form to show Roy Hodgson he should be in the England squad. I think it has been good for him to come here and there is miles more to come from him.
"He might not have realised when he took on the challenge. At Manchester City, there is degree of comfort in that."