Barry scored the opening goal as Villa cruised to a 4-1 victory in Iceland, but by playing, Barry may have damaged his chances of a move to Liverpool before the transfer window closes because he is cup-tied.
O'Neill said: "He was in the right frame of mind and wanted to play. Nothing would have stopped him so that is pleasing.
"I have no idea what will happen between now and the end of August.
"He knew the consequences of playing tonight so the decision was very much his."
But ... can you tell us ... who was the Newcastle United player who once played for TWO clubs (one being Newcastle United) in European competition (the same competition in fact) one season?
The cup-tied rule was in place, but it seems the authorities "forgot" about it.
We will give the answer on Monday.