O'Neill is planning for more squad changes next season.
"The very fact is that it's been tough all year round and that might be something to look at again. And that's exactly what I'd like to do. I think the club is capable of sustaining that," said O'Neill.
"Everton is the case in point and there is absolutely no reason why Sunderland cannot be there. Sunderland are not a top-six team by any stretch of the imagination, but we are a top-six club, in the making. And that I genuinely believe. And I'll believe that until I leave this earth."