Everton have paid around £15 million for the former Crystal Palace player who only moved to France and Lyon last year.
At six foot six inches the 23-year-old is an inch taller and two years older than the Toffees England international centre-back Jarrad Branthwaite.
Throughout most of the summer the rumour mill has been constantly filled with stories that Branthwaite was on his way out of Goodison Park, with Manchester United his strongest suitors.
So far, Everton have successfully resisted those attempts having batted away two firm bids at least from the Old Trafford club that we know of.
Both were way below the Blues own valuation of the player and after selling Amadou Onana for £50 million, it seemed as though the club might well be able to keep Branthwaite for one more season at the very least.
With O'Brien's arrival, it is inevitable that the question about the former Carlise youth player's future will be raised again.
The Toffees are still determined to keep Branthwaite with Sean Dyche being quoted saying there is no link between the new signing and his 21-year-old star's future.
Assuming that the 'Carlise Kaiser' stays on Merseyside, then will he be lining up alongside O'Brien at the start of next season?
That is a question that we cannot as yet answer with still a month or so of the transfer window left for things to change again.
However, given that he started nearly every game for Lyon last season it seems unlikely that O'Brien will be simply a reserve central defender for Everton.
Depending on the seriousness of James Tarkowski's current injury problem he may have to open the new campaign alongside Branthwaite.
At any rate, adding the Irishman to his squad makes sense providing more depth and competition with only Micheal Keane and Mason Holgate in reserve.
And after yesterday's dreadful 3-0 defeat at Coventry City with poor performances from those two in the heart of the defence, any help preventing goals seems to be needed!