The Ostrach originates on the north side of the European watershed, in the vicinity of Fleischwangen in the Landkreis Ravensburg and drains out of the Pfrunger wetlands.

It runs parallel to the Ablach for most of its length, flowing between two moraine hills by Ostrach, through the Weithart, and the Göge-Ablach plateau.

The mouth of the Ostrach lies in Hundersingen, a suburb of the community of Herbertingen in the Landkreis Sigmaringen where it runs for 22 kilometres (14 mi) to the Danube.

Tributaries of the Ostrach include the Wilhelmsdorf canal, die Fleischwanger Ach (creek), the Hornbach, the Tiefenbach and the Seebach.

In the upper range of the Ostrach, the beavers have erected a dam across the stream, creating a pond that holds 1000 cubic meters of water, causing the tributaries of the Ostrach to overflow their banks, to the great frustration of the farmers in Riedhausen and Laubbach.