The 24-year-old has joined Liverpool in a £40m deal.
And his move was confirmed earlier today on deadline day.
It brings to an end a six-year spell at the Emirates for Oxlade-Chamberlain.
The England international turned down a hefty pay rise at Arsenal to push through a move to Liverpool.
He will earn £120,000 a week at Anfield, but was offered £180,000 to stay at Arsenal.
Oxlade-Chamberlain says he wants to continue his development at Liverpool under Jurgen Klopp.
But he delivered a heartfelt message to Gunners supporters thanking them and the club for everything.
"After six years, three FA Cup wins and countless incredible memories, the time has come for me to move on from Arsenal," Oxlade-Chamberlain tweeted.
"I want to publicly thank Arsene, the coaching and club staff and my amazing team-mates for all of the incredible opportunities I have been given.
"Thank you so much to you, the fans, who have always had my back - I honestly couldn't be more grateful.
"The club, and my time here, will always mean a massive amount to me.
"I know my choice might come as a surprise to many, and the decision to leave was tough after being a part of the club for some many years.
"But I feel that this move is right for the next stage in my ongoing development."
Oxlade-Chamberlain is away on international duty but could make his Liverpool debut on September 9 at Manchester City.
