Primary intraventricular oligodendroglioma (IVO) is a rare form of clear cell neoplasm and diagnosis is challenging because other clear cell neoplasms such as central neurocytoma must be ruled out. We report a case of primary IVO in which Olig2 immunohistochemistry was useful for the diagnosis. A 33-year-old man was admitted to our hospital with severe headaches due to an intraventricular mass lesion and underwent total resection of the mass lesion. The histological diagnosis was oligodendroglioma because the tumor was negative for synaptophysin and positive for Olig2. IVO is rare and differential diagnoses must be considered. The correct pathologic diagnosis is essential for anticipating the prognosis and selecting adjuvant therapy. In this case, Olig2 was useful for the diagnosis of oligodendroglioma.