O'Riley, a Denmark international, will play for Marseille in the French league and the Champions League.
Brighton's manager, Fabian Hurzeler, said O'Riley will have the chance to play at a high level and gain more playing experience.
Hurzeler also mentioned that O'Riley will return to Brighton, where he has a long-term future, and that this move will give him more opportunities to play.
O'Riley joined Brighton from Celtic a year ago and made 24 appearances in the previous season.
