"All I can say is that there is a train in the distance and it is not going to stop," said O'Leary.
"There are nine or 10 players gone since I came here in May. In the summer another five or six can leave the club on a free transfer.
"Big decisions are going to have to be made by this club at the end of the season. Lots of things need to be changed.
"There's no getting away from the fact that there is going to be a very small squad at Aston Villa at the end of the season.
"I'd love to bring a couple of people in now. I can't.
"I love this club. Everyone told me 'Don't come here', but somebody has got to come and put this club right and, if you do, people will know the task you took on.
"I'm determined to do it. I've no regrets coming here - but big decisions have got to be made at the end of the season."