Background: Oligodendrogliomas, molecularly defined by IDH mutation and 1p19q codeletion, have a protracted natural history and are more chemoresponsive compared to astrocytomas. While the addition of chemotherapy to radiation improves survival, if chemoradiation should be initiated following diagnosis or at progression remains unknown. Given the potential for early and late radiation-related toxicity, a careful evaluation of timing of treatment is critical to optimize treatment guidelines for oligodendroglioma. We aimed to evaluate the benefit of initial adjuvant radiation therapy (RT) on survival in patients with oligodendrogliomas. Methods: We conducted a multicenter retrospective study of patients treated at the University of Washington and Stanford Medical Center with oligodendrogliomas using both 1p19q codeletion and IDH mutation to define the diagnosis, reflecting the World Health Organization 2021 diagnostic criteria. Survival probability, overall survival (OS) and time to first progression (PFS), were calculated using Kaplan-Meier method with distributions compared using log-rank test in GraphPad Prism. Propensity Score Matching was completed using XLSTAT. Results: 243 patients were identified as oligodendroglioma and 229 had treatment and survival data available with a median follow up of 5.8 years(y). Median OS was 22.5y and median PFS was 5.35y. 110 patients were treated with adjuvant radiation (aRT) within 1y of diagnosis and 119 underwent observation or chemotherapy alone (delayed RT, or dRT). PFS was comparable between aRT (7.7y) and dRT (4.6y) but OS was longer in dRT (27.8y) than aRT (13.7y) (p < 0.001). Given imbalance in baseline characteristics, propensity score matching was used to adjust for high risk factors of age, grade and presence of residual disease resulting in 63 pairs. In this matched cohort, aRT prolonged PFS (aRT = 7.75y v. dRT = 4.33y, p = 0.006) however dRT still demonstrated a survival benefit (p = 0.017). 59 dRT patients were treated with RT at progression and also had longer medan OS of 22.6y compared to 13.7y in the aRT (p = < 0.0001). The time to RT for this cohort ranged be 1.1-19y. Conclusions: To our knowledge this is the largest retrospective study of molecularly defined oligodendrogliomas that includes treatment information. This study confirms the protracted natural history of this disease. In our multi-institution cohort, early adjuvant radiation therapy may increase PFS but was not found to prolong OS. Prospective studies evaluating the timing of treatment in patients with oligodendroglioma are needed.