The Ork homeland is Orkland, where they are "the absolute rulers of all living things, from ants to elephants," and which has no known human inhabitants. Flipper mentions that it is "not far" from Oz, and the James E. Haff/Dick Martin map shows it as a small island near Hiland and Loland, right next to the Island of Civilized Monkeys. Since the Ork initially thinks that the island on which Oz is located (which he describes as "almost a continent") might be Orkland, I have to wonder if Baum thought of it as bigger than that. Then again, maybe it was just a matter of skewed perspective on the Ork's part.
The most likely derivation that I've heard for the name "Ork" is that it's basically just "stork" without the first two letters, which seems particularly likely since Baum compares the Ork's legs to those of a stork. Baum's Orks have nothing to do with Tolkien's Orcs, a term that the British author apparently took from an Old English term for a demon or ogre. There are also types of dolphins called orcs, and the name of the most famous Ork might cause a modern reader to think of dolphins, but Baum wouldn't have had this association. The Ork is first discovered by Trot and Cap'n Bill in the water, but while Orks can swim, they prefer not to.