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- Abbaye de l'Olive at Morlanwelz (Hainaut): Cistercian nuns
- Oost-Eecloo Abbey (Abdij van Oost-Eekloo) at Eeklo (East Flanders): Cistercian nuns
- Orienten Abbey (Abbaye d'Orienten) at Rummen (Flemish Brabant): Cistercian nuns
- Orval Abbey (Abbaye Notre-Dame d'Orval) (extant) at Villers-devant-Orval, Florenville (Luxembourg): Benedictine monks, then Canons Regular, then Cistercian monks, then Trappist monks
- Oudenaarde Abbey (Abdji van Oudenaarde) at Oudenaarde (East Flanders): Benedictine monks
- Oudenbourg Abbey (Abbaye Saint-Pierre d'Oudenbourg) at Oudenbourg: Benedictine monks
Read more about this topic: List Of Christian Religious Houses In Belgium