Oostum is a wierde on the territory of Garnwerd in the municipality of Westerkwartier in the Dutch province of Groningen.

The hamlet lies on the road of the city of Groningen to Garnwerd and is part of the Pieterpad.

The mound dates from the Iron Age or later and lies on an old tidal marsh bank.

Other excavations found one native Roman and dozens of pieces of medieval pottery.

It was made by Henrik Kokenbacker for the church of Feerwerd, located to the north-west of Oostum.