The likes of Steve Sidwell, Nigel Reo-Coker, Habib Beye, Nicky Shorey, Luke Young and Curtis Davies will be available to move if the price is right although they all have at least 12 months of their current contracts remaining.
But O'Neill points to Wales international Bale, who re-established himself at White Hart Lane under Harry Redknapp last season, as an example of how quickly situations can change.
He said: "Look at how Harry Redknapp has gone and re-entrusted players again like Gareth Bale. He was out of the picture at Tottenham, then came back into the side and did brilliantly.
"That sort of things happens at every club and I keep saying this to some of our players who have obviously been disappointed at not playing.
"It changes so quickly. You play really well in a game, you might get unlucky to be left out next time. But then, the second time, there's no reason to be left out and it just goes on from there."
O'Neill admits any player who makes a favourable impression will come under consideration for a place in the starting line-up despite the success of last season.
Villa finished sixth in the Premier League and reached the final of the Carling Cup and semi-finals of the FA Cup.
O'Neill said: "I think the players here last season did remarkably well. Let's take the example of James Milner. He went straight into the midfield and had a season never to forget.
"But that doesn't mean if another player steps in and does really well in the team, that he can't cement his place."
Villa will not be at full strength when they report back for pre-season training on Thursday.
The England trio of Milner, Emile Heskey and Stephen Warnock have been allowed additional time off after their involvement in the World Cup, along with United States goalkeeper Brad Guzan.