Villa are outsiders for a fourth place finish as they currently trail Manchester City by five points and a challenge for a Europa League spot is more likely.
O'Neill said: "People here at Villa have talked about fourth spot for a long time.
"Manchester City have spent in the region of £200million and for £200million there is a fair chance you are going to get some really decent players and that is exactly what they have done.
"He said that they could finish fourth or finish seventh and that was before Everton came into the picture.
"So these teams are vying for the same things as Villa and no-one has said it would be easy.
"In fact it has been exceptionally difficult. But it has been worth the trip.
"I felt at the start of the season when someone asked me if we could finish in the same position as last year (sixth) that it would be exceptionally difficult because we have to be contesting every single week, every single month."