And, even though O'Neill lost the services of key players in skipper Martin Laursen and midfielder Gareth Barry, he is confident Villa are equipped for the challenges ahead.
O'Neill said: "Am I happy with my overall business? I couldn't be more pleased. I think we have strengthened the side. It's been very pleasing indeed.
"I have got that extra defensive strength I was looking for plus Fabian Delph will be a really fine player for us - and Stewart Downing already is a fine player.
"When he is fit, he will add plenty to our squad. I think that Richard Dunne and James Collins will help us immensely. We looked a bit thin at the back.
"We used the fewest players in the league last year. That is something that we really wanted to address to try to keep players fresh.
"We've lost two really fine players in Gareth Barry and Martin Laursen. They will be hard to replace but in time we will get stronger."
O'Neill has also told left-back Nicky Shorey he has a future at the club after the collapse of his move to Portsmouth.
The former Reading player was concerned he would be frozen out at Villa after the arrival of Warnock and with Wilfred Bouma close to fitness after his lengthy injury lay-off.
O'Neill said: "Does Shorey have a part to play? Of course. I think Nicky and his agent pre-empted things a little bit by thinking he was not going to play.
"I had certainly not been pushing him out but something came up and they wanted to explore that. I gave them that opportunity but it didn't come off.
"Nicky wants to come back here and really fight for a place in the side. We have Warnock and then Freddie Bouma to come back so there's options which will be very important between now and January."
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