The former Middlesbrough and Manchester City winger, who has 12 international caps, was left out of Roy Hodgson's squad for the midweek friendly with Sweden.
The 25-year-old has struggled for consistency since making the move to the Stadium of Light over the summer, but put in arguably his best display for the Black Cats at Everton last weekend, scoring his first goal for the club in the eventual 2-1 defeat.
O'Neill believes Johnson's initial struggles were down to him having to adjust from being a regular substitute at Manchester City to being a regular starter at Sunderland.
He is confident it won't be long before Johnson shows the kind of consistent form that earned him international recognition in the first place and will therefore be pushing hard for a recall.
"I firmly believe he is a player who will come good again and regain the sort of form that got him into the England squad in the first place," the Northern Irishman told the Sunderland Echo.
"I do think it is worth bearing in mind that Adam is having to acclimatise himself to a different situation here compared to his previous club.
"For the last couple of years at Man City he has spent an awful lot of his time being used as an impact substitute coming on late in games. Here, he is being asked to play the full 90 minutes week-in, week-out.
"Over the course of time he'll get back into the groove of doing and will be all the better for it. But at the moment he has been getting through a period of adjustment and we've just had to bear with it.
"Soon, though, I'm sure we'll see Adam getting back to the sort of form which originally got him into England squads in the first place."