Everton sign Ireland centre-back O'Brien from Lyon
The 23-year-old has agreed a four-year deal for an undisclosed fee, although British media reports suggested O'Brien had moved to Merseyside for a figure of around £17 million ($22 million).
O'Brien won his first Ireland cap playing alongside Everton captain Seamus Coleman in a 2-1 win over Hungary in June.
He is the Toffees' fifth signing of the summer transfer window, following the loan return of Leeds winger Jack Harrison and the arrivals of midfielder Tim Iroegbunam and forwards Iliman Ndiaye and Jesper Lindstrom.
"Seamus has been on to me to sign for Everton," O'Brien, who left Crystal Palace for France last summer, told his new club's website. "He's a top guy, a brilliant captain and to play under him here as well as with Ireland will be a privilege.
"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.
"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."
O'Brien will now bid to force his way into a central defensive set-up where James Tarkowski and Jarrad Branthwaite were Everton's first-choice duo last season.
Branthwaite, however, has been the subject of interest from Manchester United but Everton are understood to believe their Premier League rivals have under-valued the player considerably.
"Jake further strengthens our talented core of centre-backs and adds competition to our squad which can help us continue to progress," said Everton manager Sean Dyche.
"He is still young but has earned valuable experience both abroad and at international level and has many impressive attributes to become a top-quality player for Everton in the Premier League."
Everton have made several forays into the transfer market despite the ongoing uncertainty over their ownership, with a potential sale to the Friedkin Group called off after the club failed to reach an agreement with the American investors.
It was the second time this year that Everton have been unable to push through a sale after a deal with Miami-based firm 777 Partners also broke down.