Duncan, 24, has been among the Red Bulls' most consistent players in 2021 as the club made a late surge to qualify for the Audi MLS Cup Playoffs. He's made 32 appearances (27 starts), adding a goal and three assists. He appeared in each of the club's 23 games during a pandemic-shortened season last year, with three goals and four assists. All in all, Duncan has logged 72 regular-season appearances at the club.
The Red Bulls tried to re-sign Duncan, but the fullback has long aimed for a return to Europe. Duncan had played in the Red Bull academy during his youth career but went to France to sign with Ligue 2 side Valenciennes in 2015. He never made an appearance for the first team before returning stateside with the Red Bulls in 2018.
French top-flight club Stade Reims hoped to sign him on a free transfer this winter as well, a source added. Duncan had previously been linked with the likes of Anderlecht, Metz and Montpellier this summer.
Oostende currently sit 12th in the 18-team Belgian top flight after 14 matches. MLS exports Sam Vines (Royal Antwerp), Mark McKenzie (KRC Genk), Michael Murillo (Anderlecht) and Chris Durkin (Sint-Truidense) are already playing in the Belgian league. New England Revolution breakout star Tajon Buchanan will join Club Brugge in the winter. US international Owen Otasowie plays for Club Brugge as well.
Duncan has one appearance with the US men's national team after progressing through the youth ranks.