They are both benign or malignant depending on staging.
"On scale of one to four, the World Health Organization grades tumors according to growth speed and ability to spread to nearby tissues. Depending on this grade, oligodendrogliomas are labeled either low- or high-grade.
Low-Grade (Grade II) tumors grow slowly and are benign, meaning they do not spread to nearby tissues. They are named oligodendrogliomas.
High-Grade (Grade III) tumors grow quickly and are malignant, meaning they spread to nearby brain tissue and can cause further damage. They are named anaplastic oligodendrogliomas."
https://www.neurosurgery.columbia.edu/patient-care/conditions/oligodendrogliomas#:~:text=Low%2DGrade%20(Grade%20II),They%20are%20named%20anaplastic%20oligodendrogliomas.
