The Parkhead club confirmed his exit on social media.
“We wish @Jack_Hendry2 all the best for his future career,” Celtic posted.
The centre-back spent the season on loan with the Belgian top flight side, playing 30 times and being voted by one football magazine as the best player in the country.
There was a clause in his deal which allowed Oostende to make the deal permanent.
Hendry posted a good bye message on his Instagram.
He said: "Good luck to the players and the fans @celticfc! Hopefully back there you belong next season.”
The 26-year-old, who joined Celtic from Dundee, has been at Euro 2020 with Scotland, playing in the opening game against Czech Republic.
It has been reported that Oostende could sell the player for a profit this summer with interest expected to be at a high following his impressive season.