This is an ongoing Phase 2, open-label, single-center, non-randomized study of sintilimab (one anti-PD-1 antibody same as nivolumab approved in China) plus bevacizumab administered in a low dosage schedule in adult (≥ 18 years) participants with a clinical relapse or circulating tumor DNA (ctDNA)-level relapse of Oligodendroglioma(WHO III). This study has three non-comparative study groups. Cohort 1 and Cohort 2 will receive the same study drug sintilimab 200mg and bevacizumab 3mg/kg every 3 weeks. Cohort 3 will take only standard treatment. A stringent three-step non-randomized process will be used to assign participants to one of the study groups. Neither participants nor doctors but the researcher can choose which group participants are in. No one knows if one study group is better or worse than the other. 80 total participants are expected to participate in this study (30 participants in Cohort 1 and Cohort 2). Grouping process: After enrollment, under the standard of care, participants will receive regular tumor in situ fluid (fluid within the surgical cavity, TISF) sampling for ctDNA analysis and recceive regular MRI. The researcher will study the TISF ctDNA and imaging dynamics to determine whether the tumor reaches to ctDNA-level (Cohort 1) or clinical relapse (Cohort 2). At the first step, all timely identified as ctDNA-level relapse tumors will be assigned into the Cohort 1 and receive the study drug immediately, those failed to be timely identified will be assigned into the Cohort 2 and receive the study drug after the clinical relapse. At the second step, once Cohort 1 or Cohort 2 reaches the target number, the new participants will be all assigned into the other Cohort. In the third step, if no CTDNA-level or clinical relapse was observed within 60 months after surgery, patients were assigned to Cohort 3 and further analyzed for prognostic biomarkers compared with Cohort 1 and Cohort 2.