The 24-year-old was said to be unhappy at continuously being played out of position at Arsenal by manager Arsene Wenger and turned down a move to Chelsea as he felt he would encounter the same problem in west London.
The England man has now signed a five-year contract with the Reds, worth an estimated £120,000 a week.
He completed his medical while on duty with the Three Lions before releasing a brief statement via the Liverpool website.
He 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.”
Oxlade-Chamberlain made 132 Premier League appearances for the Gunners having signed from Southampton in 2011.