His season-long loan deal at Marseille has been cut short after a stop-start stint in Provence.
Albion head coach Fabian Hurzeler will now welcome him back into a squad which lost Brajan Gruda, on loan to RB Leipzig, in attacking midfield areas.
Hurzeler said: “We’re pleased to welcome Matt back to the club. We know all about his qualities, and I have no doubt he will settle back into the group very quickly.
“He has had a good spell with Marseille, playing regular domestic football and in the Champions League, and we believe he can positively impact the team in the second half of the season.”
Marseille’s season has been thrown into turmoil by their failure to reach the knockout stages of the Champions League.
They looked well set for a play-off place before losing their last two games 3-0 to both Liverpool and Club Brugge.
Head coach Roberto De Zerbi was then reported to have offered his resignation although he denied that.
O’Riley joined OM on deadline at the end of the summer window.
He was an unused sub as Marseille let slip a 2-0 lead to draw 2-2 at Paris FC on Saturday.
The Denmark international started the season in Hurzeler’s first XI and scored in the opening day draw with Fulham from a penalty.
But he was left out after the subsequent 2-0 defeat at Everton and made his move to Marseille.
Having already played for two clubs, he cannot be loaned out to a third whose season runs from summer to spring.
Diego Coppola was on the bench for Paris FC in the Marseille game just a couple of days after completing his loan from Albion.
The expectation will be for him to enjoy plentiful game time over the coming months and return to the Seagulls for pre-season.
His colleagues now include veteran Italian goal-getter Ciro Immobile, which could offer valuable tips and insight on a daily basis on the training ground.
Immobile is joint holder of the record for Serie A goals in a single season, with 36 for Lazio, and scored 201 in the competition in total as well as helping Italy win Euro 2021.
Meanwhile Andrew Moran was poised to sign a permanent deal to Preston and end his five-and-a-half-year stint with Albion.
Much of that time has been spent on loan, most recently at LAFC in MLS.
He has also had a season in East Lancashire with Blackburn.