The Sun says O'Neill desperately needs to strengthen his squad after losing Olof Mellberg, Thomas Sorensen, Scott Carson and Patrik Berger in the summer.
Doubts also surround Shaun Maloney's Villa future while Curtis Davies and Stephen O'Halloran are still recovering from long-term injuries.
"If Gareth Barry had decided to stay we were still going to address it. Now it's even more important than ever we do it. It's all systems go."
Chelsea midfielder Steve Sidwell, 25, is expected to be one of the first arrivals while O'Neill is ready to bid £15m for Blackburn's David Bentley, 23.
O'Neill also hit back at Barry's criticism he had been more interested in Euro punditry than Villa.
He said: "It was a case of working there for two or three days then coming back and working here."