Barry was a prime target for Liverpool manager Rafa Benitez 12 months ago, but the two clubs failed to agree a fee for the England international.
The Reds were expected to renew their interest in Barry this summer, only for the 28-year-old to seal a £12million switch to cash-rich Manchester City.
Benitez claimed money had motivated Barry's decision to join City, with O'Neill revealing that Liverpool did not make a fresh offer for the former Villa captain.
O'Neill was surprised the player chose to sign for City and has questioned why a transfer to Anfield never materialised.
"Did Liverpool take it for granted that Gareth Barry would sign for them?" said O'Neill. "They may well have done. And they missed out on Barry for that reason? They may well have done.
"All I will say is that I said to Barry's agent for us not to have the same debacle as last year, that they were perfectly at liberty to speak, I assumed to the top four teams. Liverpool were aware of the price we were asking if that was the case.
"Liverpool did not contact us directly at all in this matter and never offered any money for him.
"So the impression I got was that eventually it was a straight choice.
"I am not protecting anyone here because it doesn't really matter. I said I was a bit surprised because Barry had mentioned playing in the Champions League as a reason for going, that's a person's prerogative, but Liverpool had not put a bid in."
He added: "There was an agent working, supposedly, on Liverpool's behalf, but there was no offer forthcoming. Barry left, so that was the point.
"Why Barry wanted to leave was up to him, where he went to was a bit of a surprise. But that was it."