The developmental origin of oligodendrocyte progenitors (OLPs) in the forebrain has been controversial. We now show, by Cre-lox fate mapping in transgenic mice, that the first OLPs originate in the medial ganglionic eminence (MGE) and anterior entopeduncular area (AEP) in the ventral forebrain. From there, they populate the entire embryonic telencephalon including the cerebral cortex before being joined by a second wave of OLPs from the lateral and/or caudal ganglionic eminences (LGE and CGE). Finally, a third wave arises within the postnatal cortex. When any one population is destroyed at source by the targeted expression of diphtheria toxin, the remaining cells take over and the mice survive and behave normally, with a normal complement of oligodendrocytes and myelin. Thus, functionally redundant populations of OLPs compete for space in the developing brain. Notably, the embryonic MGE- and AEP-derived population is eliminated during postnatal life, raising questions about the nature and purpose of the competition.