The Denmark international moves to Marseille on a season-long loan.
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Brighton manager Fabian Hurzeler said: "Matt has the opportunity to challenge himself at the highest level in France and in the Champions League.
"We have a lot of competition in the midfield positions, and while we see Matt's long-term future here with us, this gives him the potential of more game time."
O'Riley joined Brighton a year ago from Celtic, making 24 appearances last season.