The 24-year-old, who last week announced he would be moving on from A-League Men premiers Melbourne City after a standout season, will remain with Belgium's 10-time champions until the end of the 2026-27 campaign.
He follows Jordan Bos from City to the Belgian Pro League, with his 20-year-old former teammate's May move to KVC Westerlo earning what was described as a "club-record breaking" fee for the A-League outfit.
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O'Neill's stellar ALM form earned him a first national call-up earlier this year, making his Socceroos debut in the Sydney friendly against Ecuador in March.
He will join a long list of Socceroos to have played in Belgium, including current national team boss Graham Arnold, along with Mat Ryan, Trent Sainsbury, Jason Culina, Josip Skoko, Tony Vidmar, Paul Okon, Frank Farina and John Aloisi.
"I'm just absolutely buzzing to have it over the line and excited to get going. It's a really exciting league. Lots of good players playing there, and also progressing their careers from there.
"You can see that they're looking at Australian players now, and it's a great spot to be in. I'm just incredibly excited to start."