Inclusion Criteria:

• Histologically proven stage III colon adenocarcinoma (any T [Tx, T1, T2, T3, or T4], N1-2M0; includes N1C)
• Presence of deficient (d) DNA mismatch repair (dMMR); MMR status must be assessed by immunohistochemistry (IHC) for MMR protein expression (MLH1, MSH2, MSH6, PMS2) where loss of one or more proteins indicates dMMR; dMMR may be determined either locally or by site-selected reference lab; Note: loss of MLH1 and PMS2 commonly occur together; formalin-fixed paraffin-embedded (FFPE) tumor tissue is required for subsequent retrospective central confirmation of dMMR status
• Patients with testing that did not show dMMR (loss of MMR protein) are not eligible to participate; patients whose tumors show MSI-H by polymerase chain reaction (PCR)-based assay are not eligible to participate unless they also have MMR testing by IHC and are found to have dMMR (i.e. loss of one or more MMR proteins)
• Patients who are known to have Lynch syndrome and have been found to carry a specific germline mutation in an MMR gene (MLH1, MSH2, MSH6, PMS2) are eligible to participate
• Tumors must have been completely resected; in patients with tumor adherent to adjacent structures, en bloc R0 resection must be documented; positive radial margins are not excluded as long as en bloc resection was performed; proximal or distal margin positivity is excluded
• Entire tumor must be in the colon (rectal involvement is an exclusion); surgeon confirmation that entire tumor was located in the colon is required only in cases where it is important to establish if the tumor is a colon versus (vs.) rectal primary
• Based upon the operative report and other source documentation, the location of the primary tumor will be categorized as proximal or distal to the splenic flexure (distal includes), and further categorization will be as follows: cecum/ascending, descending, sigmoid colon, or rectosigmoid colon
• No evidence of residual involved lymph node disease or metastatic disease at the time of registration based on clinician assessment of imaging; the treating physician will determine if incidental lesions on imaging require workup to exclude metastatic disease; if based on review of images, the treating physician determines the patient to be stage III, then the patient is eligible
• No prior medical therapy (chemotherapy, immunotherapy, biologic or targeted therapy) or radiation therapy for colon cancer except for one cycle of mFOLFOX6
• Eastern Cooperative Oncology Group (ECOG) performance status =< 2
• For women of childbearing potential only, a negative pregnancy test done =< 7 days prior to registration is required; a female of childbearing potential is a sexually mature female who:
  • Has not undergone a hysterectomy or bilateral oophorectomy; or
  • Has not been naturally postmenopausal for at least 12 consecutive months (i.e. has had menses at any time in the preceding 12 consecutive months)
• Absolute neutrophil count (ANC) >= 1500 mm^3
• Platelet count >= 100,000 mm^3; platelets >= 75,000 required for patients who received cycle 1 of mFOLFOX6 prior to registration
• Creatinine =< 1.5 x upper limit of normal (ULN) or
• Calculated creatinine clearance >= 45 mL/min by Cockcroft-Gault equation
• Total bilirubin =< 1.5 x upper limit of normal (ULN) except in the case of Gilbert disease
• Aspartate aminotransferase (AST)/alanine aminotransferase (ALT) =< 2.5 x upper limit of normal (ULN)
• Thyroid-stimulating hormone (TSH) within normal limits (WNL); supplementation is acceptable to achieve a TSH WNL; in patients with abnormal TSH, if free T4 is normal and patient is clinically euthyroid, patient is eligible
• No active known autoimmune disease, including colitis, inflammatory bowel disease (i.e. ulcerative colitis or Crohn's disease), rheumatoid arthritis, panhypopituitarism, adrenal insufficiency
• No known active hepatitis B or C
  • Active hepatitis B can be defined as:
    • Hepatitis B virus surface antigen (HBsAg) detectable for > 6 months;
    • Serum hepatitis B virus (HBV) DNA 20,000 IU/ml (105 copies/ml); lower values 2,000-20,000 IU/ml (104-105 copies/ml) are often seen in hepatitis B virus e antigen (HBeAg)-negative chronic hepatitis B
    • Persistent or intermittent elevation in ALT/AST levels
    • Liver biopsy showing chronic hepatitis with moderate or severe necroinflammation
  • Active hepatitis C can be defined as:
    • Hepatitis C antibody (AB) positive AND
    • Presence of hepatitis C virus (HCV) RNA
• Excluded if known active pulmonary disease with hypoxia defined as:
  • Oxygen saturation < 85% on room air, or
  • Oxygen saturation < 88% despite supplemental oxygen
• No grade >= 2 peripheral motor or sensory neuropathy
• Patients positive for human immunodeficiency virus (HIV) are eligible only if they meet all of the following:
  • A stable regimen of highly active anti-retroviral therapy (HAART)
  • No requirement for concurrent antibiotics or antifungal agents for the prevention of opportunistic infections
  • A CD4 count above 250 cells/mcL, and an undetectable HIV viral load on standard PCR-based tests
• No other planned concurrent investigational agents or other tumor directed therapy (chemotherapy, radiation) while on study
• No systemic daily treatment with either corticosteroids (> 10 mg daily prednisone equivalents) or other immunosuppressive medications within 7 days of registration
• No known history of severe allergic anaphylactic reactions to chimeric, human or humanized antibodies, or fusion proteins
• No known hypersensitivity to Chinese hamster ovary (CHO) cell products or any component of the atezolizumab formulation
• No known allergy to 5-fluorouracil, oxaliplatin, or leucovorin