The 21-year-old has played three times for the Seagulls in the EFL Trophy, including his debut against Dons at Stadium MK in November 2018.
O'Hora, who joined the Premier League club from Bohemians in January 2018, has represented the Republic of Ireland at Under-18s and Under-19s level.
"Warren is someone we've liked for a long time," said boss Russell Martin.
"From our time watching under-23s football, he's one of the best we've seen."
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