Nivolumab or Nivolumab and Azacitidine in Patients With Recurrent, Resectable Osteosarcoma

Inclusion Criteria: * Participants must have had a histologic diagnosis of osteosarcoma at original diagnosis * Disease Status: Patients with an isolated pulmonary recurrence of osteosarcoma can be enrolled on this study. * Any history of metastatic disease at a site other than lung would make the patient ineligible for this study. * The patient's treating team must consider the patient's disease to be resectable and the patient must be willing to undergo resection of all disease, including any lung lesion meeting criteria for likely metastatic disease, defined as: 3 or more lesions ≥ 3 mm in diameter OR a single lesion ≥ 5 mm. * Patients with bilateral disease are eligible provided their disease is considered resectable. Resectable pulmonary nodules are defined as nodules that can be removed without performing a pneumonectomy (e.g., nodules immediately adjacent to the main stem bronchus or main pulmonary vessels). * Must have a performance status corresponding to Eastern Cooperative Oncology Group (ECOG) scores of 0, 1 or 2, using the Karnofsky scale for patients \> 16 years of age and the Lansky scale for patients ≤ 16 years of age * Prior Therapy: Participants must have fully recovered from the acute toxic effects of all prior chemotherapy, immunotherapy, or radiotherapy prior to the start of protocol therapy. * Participants must have normal organ and marrow function within 7 days of starting protocol therapy * All participants and/or their parents or legal guardia…