Premier League - Arsenal sign English teenager
The 17-year-old underwent a medical with the Gunners before completing a "long-term" move to the Emirates Stadium, according to the North London club, although they did not disclose the length of his contract.
The reported fee consists of an initial £6.5m with a further £5.5m to follow in performance-related add-ons.
"We are delighted that Alex has decided to join us," said Arsenal manager Arsene Wenger.
"He is an exciting young player who will provide us with creativity and offensive quality. Alex is a versatile player who can play in a number of positions. He can play as an attacking centre midfielder, or wide left and right.
"Alex is a perceptive passer of the ball and has a great understanding of the game and looks to be a very good team player.
"Alex has done extremely well during his development with Southampton and we are looking forward to him progressing even further and fulfilling his huge potential with Arsenal Football Club."
Oxlade-Chamberlain, who turns 18 next Monday, said: "I'm so excited to be joining Arsenal. It's a fantastic club and I'm really looking forward to the opportunity of playing football at the highest level.
"Arsenal have a great manager in Arsene Wenger, top-quality players and an attractive style of play. These were all important factors in me choosing to sign for Arsenal.
"I'm now looking forward to meeting all my new team-mates and giving absolutely everything in an Arsenal shirt."
The prodigious winger rocked the south-coast club last month when he revealed he wanted to move to the Premier League and he has now got his wish.
Chamberlain made 43 appearances for Saints, scoring 10 goals and helping them to promotion back to the Championship last season.
He follows in the footsteps of Theo Walcott, who left St Mary's to join the Gunners for a similar fee in 2006.