O'Riley, who was born in England, plays for Denmark and has four caps to his name. He signed for Brighton last summer from Scottish giants Celtic and made 21 appearances in the Premier League, registering two goals and two assists. Marseille were in the market for a new midfielder after deciding to sell Rabiot following his altercation with Jonathan Rowe.
O'Riley had been linked with a move away from Brighton all summer, with Juventus eager to sign him at one point.