O'Brien returns to England after a year away following his sale by Crystal Palace. He has signed a deal to 2028.
"I’m buzzing and so happy to have signed for Everton,” O’Brien told evertontv. “I've been dreaming of this kind of move in the Premier League, with such a historic club. It's a very exciting move for me.
“Everton is a huge club with a lot of history behind it and there's a big future as well.
“When the transfer window opened there were other clubs in for me but there was only one club I wanted to go to – it was Everton.
“They’ve shown the faith in me so now it's my turn to show why on the pitch.”
He added: “I'm a ball-playing centre half, fast, I like to attack in both boxes and I like to bring a bit of excitement.
“You see how Sean Dyche develops young centre-halves and what he has done for Jarrad (Branthwaite). The way he’s going now, the season he’s had in the Premier League, it’s obviously something I look at and something I want to replicate.
“This season, I just want to play as many games as I can and help the Club get higher up the table. It’s an exciting time and it’d be good to end the season – the last season at Goodison Park – as high up the table as we can.
“We’ve then got the new stadium, which looks great. Not just that but we’re shaping a team for the future and can push on.”
On the influence of his Republic of Ireland and now Everton captain Seamus Coleman, O'Brien revealed: “Seamus has been on to me to sign for Everton! He’s a top guy, a brilliant captain, and to play under him here as well as with Ireland will be a privilege.
“He’s the best captain I've ever played under. He's massively respected – not just here but in the football world in general.
“I've met all of the lads in the squad and you can sense straight away that it's a tight-knit group. Obviously, having Seamus here will help me as well.”