The 19-year-old midfielder made his first-team debut for the Gunners last season but has now completed a move to Turkey following a medicalBesiktas have announced the signing of Arsenal youngster Oguzhan Ozyakup for a fee of €500,000 (£405,000).
The 19-year-old midfielder joined the Gunners as a trainee in 2008 and made his first two senior appearances last season - in a League Cup clash with Shrewsbury and the 8-2 Premier League loss to Manchester United.
"There are many experienced players at this club and I want to learn from them. It's a significant advantage for me," he told Besiktas' official website after signing.
The Turkish giants announced that their new signing is set to receive a €100,000 (£81,000) signing-on fee, a per-game bonus of €7,500 (£6,100) limited for 40 games, and wages of €400,000 (£324,000) in his first season.
These will rise to €425,000 (£344,000) in his second, €450,000 (£364,000) in his third and €475,000 (£384,000) in his fourth season at the club.
Oguzhan has represented the Netherlands, the country of his birth, at youth international level, but has this year declared his intention to represent Turkey at international level.