The participants of this study will be children, adolescents, and young adults with residual osteosarcoma, which cannot be removed completely through surgery. Participants will have achieved a partial response or stable disease at the end of conventional chemotherapy. Osteosarcoma is cancer of the bone. The cancer cells make immature bone cells, known as osteoid. Osteosarcoma is very rare, but it is the most common type of bone cancer in children and teens. It is most common in teens and young adults. In this study, participants will receive either cabozantinib and best supportive care or the best supportive care alone. Best supportive care will be provided at the investigator’s discretion and according to institutional guidelines. It includes antibiotics, nutritional support, correction of metabolic disorders, optimal symptom control and pain management (including radiotherapy), etc. but does not include tumor specific therapy. Cabozantinib will be taken by mouth (orally), as a tablet, once a day. Cabozantinib will be provided to participants who tolerate it for as long as their disease does not progress. Participants in the study receiving best supportive care alone may switch to treatment with cabozantinib and best supportive care if their disease progresses and if other eligibility criteria are met. Participants may withdraw consent to participate at any time. The estimated duration of the study for participants is 24 months, however a participant could remain in the study longer if demonstrating treatment benefit.