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

Treatment will be administered in 28 day cycles with the first cycle in the neoadjuvant setting. This will be followed by surgery to render the participant in surgical remission. Subsequently the participant will continue to receive treatment for up to 12 additional cycles or until recurrence, whichever occurs first. For participants with known bilateral lung recurrence, the nodule\[s\] in one lung should be resected, prior to the first cycle of chemotherapy. This trial was initially designed as a Phase 1/2 study; However, phase 2 will not proceed.