Fines of €250 each were imposed for breaking restrictions on gatherings as police in the town of Hasselt, around 60km from Liege, forced them to disperse.
Oulare confirmed the incident but told Belgian outlet Het Nieuwsblad that the police were being overzealous.
"We were not holding a party but apparently the neighbours called the police," Oulare told the newspaper.
"I don't think I did anything wrong. I understand the police are doing their job but they mustn't overdo it."
Oulare, who has played for Belgium at under-21 level and was a teenage prodigy at Club Brugge, signed for Watford in 2015 but played just twice for the Premier League club before going back to Belgium.
Belgium has been hard hit by the coronavirus pandemic, with around 3,600 deaths.