The function of oligodendrocyte is -
The principal function of oligodendrocytes is the production and maintenance of myelin in CNS - Robbins

Myelination in Central nervous system → Oligodendrocytes
Myelination in Peripheral nervous system → Schwann cells
Myelinates the CNS
Nutrition of nervous tissue
Lining the cavities of the CVS
Behave like macrophages