The Liverpool Twitter account relayed news of the midfielder's departure from the Emirates Stadium:
Given he is away with England on international duty, Oxlade-Chamberlain didn't have too much to say, per Mike Anstead of the MailOnline:
Liverpool manager Jurgen Klopp expressed his delight at getting the signing through the door, per the club's official website:
I remember the first time I saw him play live - it was at Dortmund in 2014 and he came on as a sub that day and made a big impact. He stood out immediately because of his pace and skills and his attitude in such a big game for a young player; I remember him chasing back to make a brilliant challenge on Mkhitaryan in the box to save a certain goal.
We won that game but lost the return match at Arsenal when he started and played well, I think he hit the post. Since then I have followed him and when I heard it may be possible to sign him I didn't need to think twice.
The 23-year-old will be looking to get more game time at Liverpool having failed to nail down a first-team spot at Arsenal since his move to the Emirates Stadium from Southampton in 2011.
The England international told Sky Sports' Goals on Sunday show back in October 2016 that he was unhappy with his lack of game time with the Gunners, per Sky Sports' Patrick Surlis.
In 2016-17 he played a part in 29 of Arsenal's Premier League matches, but 13 of those appearances came from the bench, WhoScored.com.
Oxlade-Chamberlain was long touted as the next big thing at the Emirates, but he has failed to make good on his huge potential.
A combination of injury, poor form and competition has often seen Oxlade-Chamberlain forced to the sidelines, something he will hope to change at Liverpool. He remains a talented player, and he still has potentially the best part of a decade to reach his prime.
If manager Jurgen Klopp uses him correctly, the Reds could have bagged themselves a terrific signing. Bleacher Report's Rob Blanchette thinks he'll thrive under his new boss:
It has long been reported that the German boss is a big fan of Oxlade-Chamberlain's, specifically his "energy, explosiveness and ability to perform a variety of attacking roles," per the Sunday Times' Jonathan Northcroft.
Oxlade-Chamberlain can be an effective player. He is not afraid to take on defenders and could be devastating used in the right system.