O'Riley joined Marseille on a season-long loan last summer before his stay in France was mutually terminated early in February after scoring just once in 25 appearances.
The Danish international has since featured sporadically for Fabian Hurzeler's side, playing just six times across 206 minutes, with Brighton pushing for a place in Europe.
Brighton are currently seventh in the Premier League, and if they win their final game against Manchester United on Sunday, they could finish as high as sixth if Bournemouth drop all three points against Nottingham Forest.
Whilst the Seagulls have Europe in their own hands, O'Riley's future at the club is uncertain, and according to Sky Germany, Borussia Dortmund are interested in the 25-year-old.
Dortmund boss Nico Kovac finds the midfielder an "intriguing" prospect, with Brighton setting his valuation between €15 and €20 million.
O'Riley is under contract at the AMEX Stadium in 2029, but with first-team football on the South Coast limited, the Bundesliga could be an ideal destination.