• Participants must have had histologic verification of osteosarcoma at the original diagnosis or relapse

• Participants must be in first or greater relapse of osteosarcoma

• Recurrence must be limited to the lung but can be unilateral or bilateral

• All pulmonary nodules must be resectable as determined by the institutional surgeon. 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). There is no maximum number of lesions provided the surgeon thinks a complete surgical remission can be achieved.

• Participants must have at least 1 lesion that is ≥ 5 mm and meets the criteria to receive SBRT AND an additional nodule(s) that meets protocol definition for a metastatic nodule necessitating surgical resection: single nodule ≥ 5 mm, or ≥ 2 nodules ≥ 3 mm in size

• Patients must have a Lansky (≤ 16 years) or Karnofsky (\> 16 years) score of ≥ 60, or ECOG performance score of ≤ 2

• All prior treatment-related toxicities must have resolved to ≤ Grade 1 OR be determined clinically stable by the treating investigator.

‣ Myelosuppressive chemotherapy: ≥ 14 days after the last dose of myelosuppressive chemotherapy.

⁃ Hematopoietic growth factors: ≥ 14 days after the last dose of a long-acting growth factor (e.g., Pegfilgrastim) or 7 days for a short-acting growth factor.

⁃ Biologic (anti-neoplastic) agent: ≥ 7 days after the last dose of a biologic agent.

⁃ Cellular therapy: ≥ 21 days must have elapsed from the last dose of any type of cellular therapy (e.g., modified T cells, NK cells, dendritic cells, etc.) with resolution of any associated toxicities.

⁃ Interleukins, interferons, and cytokines (other than hematopoietic growth factors): ≥ 21 days must have elapsed from the last dose of interleukins, interferon, or cytokines (other than hematopoietic growth factors).

⁃ Antibodies: 7 days or 3 half-lives (whichever is longer) but not longer than 30 days, and toxicity related to prior antibody therapy must be recovered to Grade ≤ 1.

⁃ Autologous Stem Cell Transplant or Rescue: ≥ 6 weeks must have elapsed since stem cell transplant or rescue.

⁃ Radiotherapy (XRT): ≥ 14 days after local palliative XRT (small port); ≥ 3 months must have elapsed if prior craniospinal XRT was received, if ≥ 50% of the pelvis was irradiated, or if TBI was received; ≥ 6 weeks must have elapsed if other substantial bone marrow radiation was given.

⁃ Investigational Agents Not Otherwise Specified: ≥ 28 days must have elapsed since the last dose of any investigational agent not specified above.

‣ Thoracic Surgery or Procedure: ≥ 28 days must have elapsed since prior thoracotomy, thoracoscopy, or thoracentesis.

• Adequate Bone Marrow Function Defined: Peripheral absolute neutrophil count (ANC) ≥ 750/mm3, Platelet count ≥ 50,000/mm3. Must be transfusion independent defined as not receiving platelet transfusions for at least 7 days before enrollment

• Adequate Renal Function Defined As Creatinine clearance or radioisotope ≥ GFR 70ml/min/1.73 m2

• Adequate Liver Function Defined As Total bilirubin ≤ 1.5 x the upper limit of normal (ULN) for age, ALT (SGPT) ≤ 3 x the ULN. For this study, the ULN for ALT (SGPT) is 45 U/L.

• Adequate Pancreatic Function Defined As Serum lipase ≤ 1.5 x ULN

• Adequate Thyroid Function Defined As Normal free T4

• Adequate Pulmonary Function Defined As No dyspnea at rest, Pulse oximetry \> 92% on room air

• Adequate Cardiac Function Defined As QTc ≤ 480 msec, Shortening fraction ≥ 27% by echocardiogram or ejection fraction ≥ 50% by gated radionuclide study or echocardiogram

• Urine protein: Meets one of the following criteria: (1) urinary protein by urine dipstick is ≤ 100 mg/dL or ≤ 2+; OR (2) Urine Protein Creatinine (UPC) ratio \< 3.5; OR (3) if 24-hour urine protein was measured, urinary protein ≤ 3500 mg.

• Life expectancy of at least 4 months.

• Negative urine or serum pregnancy test in women of childbearing potential.

• Sexually active patients of reproductive potential are not eligible unless they have agreed to use an effective contraceptive method for the duration of their study participation. Patients should maintain adequate contraception for at least 5 months after the last dose of atezolizumab. Adequate contraception is defined as abstinence or use of contraceptives with a failure rate of \< 1% per year.

• All participants and their parents or legal guardians must sign a written informed consent and assent (if applicable).