Oleksandr is a versatile Ukranian who played for Manchester City.
The 25-year-old flew back from City's pre-season tour of the USA to complete the move after the north London club agreed a £30 million deal.
He has signed a long-term contract with the club.
Mikel Arteta said that Alex is a high quality player.
I followed him after I left Manchester City, I know him well.
It's not just about the positions he can play but also how versatile he will be.
Everyone has made a huge effort to bring Alex to the club, and I'm happy that he's here.
The player who played many games at left-back for the Blues only appeared in 15 games last season.
He is the second player to leave City this year.
Gabriel Jesus joined the team for £45 million.
The new player joined his teammates at the pre-season camp in Florida.
The squad number will be announced soon.
The technical director said that Oleksandr was a main focus.
Alex's attributes will put our squad at a different level and we're happy to have him with us now.