Brief Summary: This phase I trial studies the side effects of BO-112 when given together with nivolumab before surgery in treating patients with soft tissue sarcoma that can be removed by surgery (resectable). Immunotherapy with monoclonal antibodies, such as nivolumab, may help the body's immune system attack the cancer, and may interfere with the ability of tumor cells to grow and spread. Immunotherapy with BO-112, may induce changes in body's immune system and may interfere with the ability of tumor cells to grow and spread. Giving nivolumab and BO-112 before surgery may work better in treating patients with soft tissue sarcoma compared to nivolumab alone.

Detailed Description: PRIMARY OBJECTIVE: I. To determine the safety and tolerability of integrating BO-112, and BO-112 with nivolumab in soft tissue sarcoma patients undergoing preoperative radiotherapy. SECONDARY OBJECTIVES: I. Determine the change in T cell infiltration at surgical resection from baseline as measured by immunohistochemistry. II. Assess treatment effect (necrosis score) of BO-112 and BO-112 in combination with nivolumab in soft tissue sarcoma patients receiving preoperative hypofractionated radiotherapy, compared to patients treated on a recently completed phase 2 study of preoperative hypofractionated radiotherapy alone. III. Evaluate the 2-year rate of local and distant metastasis of BO-112 and BO-112 in combination with nivolumab in patients with localized resectable soft tissue sarcoma receiving preoperative hypofractionated radiotherapy, as compared to historical patients receiving preoperative radiotherapy alone. EXPLORATORY OBJECTIVES: I. Evaluate the dynamics of the intratumoral T cell phenotype. II. Assess the capacity to grow ex vivo tumor infiltrating lymphocytes from patients treated with BO-112 and BO-112 in combination with nivolumab. III. Analyze changes in T-cell receptor (TCR) repertoire in tumor infiltrating lymphocytes and peripheral blood mononuclear cells (PBMCs) between baseline, post-treatment, and ex vivo cultured tumor infiltrating lymphocyte (TIL) samples. IV. For patients with disease burden outside of the resected lesion, assess imaging response to treatment using Response Evaluation Criteria in Solid Tumors (RECIST) criteria. OUTLINE: Patients receive BO-112 intratumorally on days 8 and 15 or 1, 8, and 15 and nivolumab intravenously (IV) over 30-60 minutes on day 8 in the absence of disease progression or unacceptable toxicity. Patients also undergo standard of care radiation therapy on days 8-12 for a total of 5 fractions. Patients then undergo standard of care definitive surgical resection on day 26 to 50. After completion of study treatment, patients are followed up at 2 weeks, 3 months, and between 6-12, 12-18, and 18-24 months.