The England international penned a long-term contract with the Reds after a medical was conducted at St George’s Park, where the 24-year-old is currently on duty with his country.
Oxlade-Chamberlain made 132 Premier League appearances for the Gunners during his six years in north London, and has 27 caps for the Three Lions to date.
The midfielder, the club’s fourth signing of the summer, bolsters Jürgen Klopp’s squad as Liverpool prepare to compete in three domestic competitions and the Champions League.
In a message to Reds fans after sealing the move, Oxlade-Chamberlain said: “I’m currently away with England, so I’m at St George’s Park. Firstly, I’m delighted to have signed for Liverpool and a big thank-you to everyone here at the FA that has made this possible, using the facilities. It has been a massive help.
“I’m not going to say too much today; I’m here with England and we’ve got two big games coming up, so I want to focus on them. But I can’t wait to get to Melwood and you’ll hear first from me on LFCTV.”