"It's a realistic possibility but we are not setting targets, our next target is Romania, simple as that," O'Neill told Sky Sports News.
"We know they are an excellent team and they top the group, so we will have to be at our very best to beat them. But we are capable of doing that and the players believe that.
"With every game they are starting to believe more and more and it's a game we can look forward to."
Northern Ireland meet first-placed Romania in June.