Oligoastrocytoma: A type of malignant brain tumor that belongs to the category of "mixed" gliomas. Gliomas are brain tumors that begin in glial, or supportive, cells in the brain or spinal cord. Malignant gliomas are the most common primary tumors of the central nervous system (the brain and spinal cord). Mixed gliomas are gliomas that contain more than one type of cell. An oligoastrocytoma is a mixed glioma composed of cells that resemble two different kinds of brain cells known as astrocytes and oligodendrocytes.