The O is a rivulet in Lower Sodden. It, along with the A, is a tributary of the Chotla which begins in Mahakam.
In Chrzest Ognia (Baptism of Fire), there is an amusing little exchange between Zoltan's company and Dandelion about the name of the rivulet. When the name is explained, the poet responds with an understanding "Ah!", to which the dwarves reply, "No! That's a different river. The A is to the north".