The 24-year-old Dane arrived in Marseille yesterday, to finalise a move where Olympique Marseille is set to cover his full wages with a €2 million loan fee plus add-ons.
The Danish international midfielder will now spend the rest of the campaign in France under Roberto De Zerbi after featuring twice in the Premier League for Brighton this season.
Following the departure of Adrien Rabiot to AC Milan, O’Riley is set to bolster OM’s midfield options after de Zerbi's struggling side suffered their second defeat in three Ligue 1 matches against Lyon on Sunday.
O’Riley, who was made his debut in the Danish national team, was a standout performer at Celtic before joining Brighton last season. O’Riley made 23 Premier League appearances last season, and scored three goals in the process.