The 29-year-old midfielder was released by Arsenal this summer after nine injury-plagued seasons.
Diaby made just two appearances for Arsenal in the past two seasons and has 16 caps for the Turkish national side.
Özil also suffered injury last season keeping him out from October to January. The 26-year-old has been in good form since recovering from his injury winning two Vitality Arsenal Player of the Month and a PFA Fans Player of the Month award.
The former Real Madrid man has already won three trophies this summer including the FA Cup, the Barclays Asia Trophy and most recently the Emirates Cup. Arsenal could add to their silverware next week when they take on Chelsea in the Community Shield.
Özil provided nine assists and five goals in 32 appearances for Arsenal in all competitions last season.