Olig1 and Olig2 are bHLH transcription factors expressed in neural progenitors and are essential for oligodendrocyte development. Olig2 is required for oligodendrocyte and motor neuron specification in the spinal cord, while Olig1 is involved in oligodendrocyte development and maturation, particularly in the brain. Both genes contribute to neural pattern formation, but neither is required for astrocyte development. Fate mapping analysis shows that oligodendrocytes arise from Olig-specified progenitors that also give rise to neurons. Olig2 is essential for motor neuron specification in the spinal cord, while Olig1 is not required for motor neuron development but promotes oligodendrocyte formation and maturation. Olig1 and Olig2 are coexpressed in progenitors that give rise to motor neurons and oligodendrocytes, but Olig1 can compensate for Olig2 loss in some contexts. Olig2 is required for motor neuron and oligodendrocyte development in the spinal cord and hindbrain, and its absence leads to the loss of motor neurons and oligodendrocytes. Olig2 also regulates the pMN domain, which is crucial for motor neuron and oligodendrocyte development. Olig1 is expressed in progenitors that give rise to motor neurons and oligodendrocytes but not astrocytes. Olig2 is required for the specification of all motor neurons, and its absence leads to the failure of motor neuron development. Olig1 and Olig2 have distinct roles in the regulation of the pMN domain, with Olig2 acting as a repressor of V2 interneurons. Olig function is essential for oligodendrocyte precursor development, independent of motor neuron specification. Olig1 has unique biological functions, including promoting oligodendrocyte precursor development in the brain of Olig2 null mice. Olig1 and Olig2 are structurally related, with Olig2 being more conserved. Olig1 may have arisen from Olig2 via gene duplication. Olig1 and Olig2 are involved in neural pattern formation and the maintenance of the pMN domain. Their functions are essential for the development of motor neurons and oligodendrocytes, and their roles in cell fate specification are critical for CNS development.Olig1 and Olig2 are bHLH transcription factors expressed in neural progenitors and are essential for oligodendrocyte development. Olig2 is required for oligodendrocyte and motor neuron specification in the spinal cord, while Olig1 is involved in oligodendrocyte development and maturation, particularly in the brain. Both genes contribute to neural pattern formation, but neither is required for astrocyte development. Fate mapping analysis shows that oligodendrocytes arise from Olig-specified progenitors that also give rise to neurons. Olig2 is essential for motor neuron specification in the spinal cord, while Olig1 is not required for motor neuron development but promotes oligodendrocyte formation and maturation. Olig1 and Olig2 are coexpressed in progenitors that give rise to motor neurons and oligodendrocytes, but Olig1 can compensate for Olig2 loss in some contexts. Olig2 is required for motor neuron and oligodendrocyte development in the spinal cord and hindbrain, and its absence leads to the loss of motor neurons and oligodendrocytes. Olig2 also regulates the pMN domain, which is crucial for motor neuron and oligodendrocyte development. Olig1 is expressed in progenitors that give rise to motor neurons and oligodendrocytes but not astrocytes. Olig2 is required for the specification of all motor neurons, and its absence leads to the failure of motor neuron development. Olig1 and Olig2 have distinct roles in the regulation of the pMN domain, with Olig2 acting as a repressor of V2 interneurons. Olig function is essential for oligodendrocyte precursor development, independent of motor neuron specification. Olig1 has unique biological functions, including promoting oligodendrocyte precursor development in the brain of Olig2 null mice. Olig1 and Olig2 are structurally related, with Olig2 being more conserved. Olig1 may have arisen from Olig2 via gene duplication. Olig1 and Olig2 are involved in neural pattern formation and the maintenance of the pMN domain. Their functions are essential for the development of motor neurons and oligodendrocytes, and their roles in cell fate specification are critical for CNS development.