Oostzaan is a municipality and a town in the Zaanstreek, Netherlands, in the province of North Holland. The municipality had a population of 9,757 in 2019. Oostzaan has a total area of 16.08 km² of which 4.55 km² is water.
Oostzaan—together with Westzaan and Assendelft—are considered the "mother towns" of the Zaanstreek, of which they are the three oldest towns. Oostzaan also played a role in the VOC and WIC shipping and shipbuilding.
In the 17th century Oostzaan had its own pirate, named Claes Compaen. Originally sailing from the Netherlands as a legal pirate captain, in the possession of pirate letters, he soon began to raid ships for his private account, from the English Channel, to the Mediterranean Sea and the African-Atlantic coast into even the Caribbean Sea. He liquidated the booty on the coast of Ireland, later in Salé and the Barbary Coast. A street in Oostzaan is named for Claes Compaen.
The town has a Reformed church with a cruciform groundplan, the Great Church, which contains two ship models that recall the days when Oostzaan was an important sea-faring community. In the Oostzijderveld area of Oostzaan stands a windmill, De Windjager.