Cahill, a product of the Villa academy, has flourished at the Reebok Stadium and forced his way into the England squad, though he was turned over on Saturday by John Carew.
O'Neill said: "Have I any regrets that Cahill left? No.
"He wasn't exactly kicked out of the door, but Gary got to the stage where he wanted to play first-team football.
"He felt he wasn't going to get that in the coming weeks or months on a regular basis at Villa and, if we couldn't keep him, then we don't really want to stop people from making progress.
"Bolton, Birmingham and Sheffield United were interested. Bolton paid us £5million for the lad plus the sell-on fee and we were happy to do it.
"He was happy to go, we are happy and we still remain happy with the situation.
"Gary wanted to play regularly somewhere and Bolton gave him that chance."