He said: "As soon as I knew that Middlesbrough were interested I was keen to come up here for talks. I was impressed at what they are trying to do here with a young squad. I can't wait to get started.
"It's a really good set-up. I can't see any reason why we can't challenge for a place in the top 10 this season."
O'Neil has been training on Teesside for a week now, and is getting his feet under the table.
He said: "The two weeks' break has been useful because it's given me the chance to get used to the lads. I have got involved in the team spirit and I have really enjoyed it.
"It's not easy moving when you have been at your previous club for such a long time. It can be a bit strange. But training has been good and everybody has made me very welcome.
"Obviously quite a few of the lads are away on international duty so I'll get to meet them all by the end of the week."