The England international has penned a long-term contract at Anfield after completing a £35m switch from the Emirates.
Chelsea also agreed a fee with the Gunners, but the 24-year-old turned down the champions in order to join Liverpool.
After learning of the news, Ramsey, who has played alongside Oxlade-Chamberlain for the last six years, took to Instagram to write: “Another of our British boys heading on to new things.
“Not only have we lost a great player we’ve lost a great guy in the changing room and It is sad to see you go mate. Had some great memories over the years playing along side you and I wish you all the best with this new chapter in your life #theOX.”