INCLUSION CRITERIA: Participants must have histologically or cytologically confirmed primary diffuse large B- cell lymphoma of the CNS (PCNSL) or an aggressive B-cell lymphoma with secondary involvement of the CNS (SCNSL). NOTE: Participants with B-cell lymphomas that were previously indolent but now involve the CNS (i.e., transformed from previous follicular lymphoma, chronic lymphocytic leukemia, marginal zone lymphoma, and mantle cell lymphoma) are eligible. Participants must have a disease that is relapsed or refractory after initial systemic treatment or participants without prior therapy who are considered ineligible for standard frontline therapy Participants must have the evaluable disease by clinical exam (i.e., palpable lymphadenopathy, measurable skin lesions, etc.) and/or imaging (measurable lymph nodes or masses on CT or MRI and/or evaluable FDG avid lesions on PET) Participants with second malignancies not requiring active systemic therapy or pre- malignant conditions such as monoclonal B-cell lymphocytosis (MBL) or monoclonal gammopathy of undetermined significance (MGUS). Participants that are positive for hepatitis B core antibody, hepatitis B surface antigen (HBsAg), or hepatitis C antibody must have a negative hepatitis B and/or C viral load by polymerase chain reaction (PCR). Participants with suspicious radiologic evidence of aspergillosis infection (i.e., Chest CT and/or Brain MRI) are eligible if confirmatory laboratory testing of Beta-D glucan and aspergillus antigen are negative. Age >=18 years ECOG performance status <=2. NOTE: In participants with neurologic deficits caused by CNS lymphoma any ECOG status is acceptable to be eligible.  Participants must have adequate organ and marrow function as defined below:  absolute neutrophil count >=1000 cells/mcL (1 x 10^9/L) platelet count >= 50,000 cells/mcL (50 x 10^9/L) hemoglobin > 8.0 g/dL (transfusions permitted) total bilirubin <= 1.5 x upper limit of normal (ULN) (unless Gilbert s syndrome or disease infiltration of the liver is present) Aspartate Aminotransferase or serum glutamic oxaloacetic transaminase/ Alanine Aminotransferase or serum glutamic pyruvic transaminase AST(SGOT)/ALT(SGPT) <= 5.0 x institutional ULN unless lymphoma related Serum Creatinine OR creatinine clearance (Cr Cl) <= 1.5 mg/dL OR >= 40 ml/min/1.73m^2  NOTE: Cr Cl will be calculated with the use of the 24-hour creatinine clearance or eGRF in the clinical lab  Laboratory assessments to determine eligibility may be performed at CLIA (or equivalent) certified laboratories outside the NIH and results forwarded to the study team for review and management . Given that the methodologies utilized are similar across all laboratories, no significant variability is expected and there is no anticipation that study data will be affected. However, as different laboratories use slightly different kinds of equipment, each laboratory must determine/validate its own reference ranges. Therefore, on this protocol, normal ranges from each lab will be used in reference to terms such as ULN, except in cases where absolute values are appropriate and are specified as such  Prothrombin time (PT) and activated partial thromboplastin time (aPTT) must be < 1.5 x the ULN; except if, the aPTT is prolonged because of a positive Lupus Anticoagulant.  Male and female participants must agree to use certain methods of birth control. A highly effective method of birth control for female participants is defined as a method that has a low failure rate (i.e., less than 1% per year) when used consistently and correctly and includes implants, injectables, birth control pills with two hormones, some intrauterine devices (IUDs). Male participant cannot use highly effective methods and are required to use barrier. The specific guidelines are as follows:  Women: Females of childbearing potential (FOCBP) , defined as a sexually mature female who: 1) has not undergone a hysterectomy or bilateral oophorectomy; or 2) has not been naturally postmenopausal (amenorrhea following cancer therapy does not rule out childbearing potential) for at least 24 consecutive months (i.e., has had menses at any time in the preceding 24 consecutive months), must either commit to continued abstinence from heterosexual intercourse or begin TWO acceptable methods of birth control, one highly effective method and one additional effective method AT THE SAME TIME, at least 28 days before she starts taking Revlimid, as well as for the duration after the last dose of study drug as listed below. Men: Men must agree to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures to prevent pregnancy of their partner and should also agree to not donate sperm while taking the study treatment and for the durations as listed below.  Contraception Requirements:  Pre-Treatment/During Treatment (Women/Men - Time frame prior to/during dosing):  --All drugs : Women Begins 28 days prior to treatment; Men Begins on day 1  Post-Treatment (Women/Men - Time frame after the last dose):  Venetoclax - Women 90 days; Men 90 days Ibrutinib - Women 3 months; Men 3 months Obinutuzumab - Women 18 months; Men 6 months Revlimid - Women 28 days; Men 28 days Nivolumab - Women 5 months; Men 5 months All study participants must be registered into the mandatory Revlimid REMSTM program and be willing and able to comply with the requirements of Revlimid REMSTM. Breastfeeding participants must be willing to discontinue breastfeeding from study treatment initiation through designated time points after study drugs discontinuation (as required for women contraception)  EXCLUSION CRITERIA:  Participants with plasmablastic lymphoma and B-cell lymphoma, unclassifiable, with features intermediate between DLBCL and classical Hodgkin lymphoma are not eligible Chemotherapy (excluding corticosteroids), radiation therapy, and/or monoclonal antibody <= 7 days prior to first administration of study treatment. Rationale for a short 7- day window is that participants with relapsed CNS lymphomas often have existing neurologic conditions that mandate urgent therapy. Previous treatment with more than one of the following classes of medications: (1) BTK inhibitors, (2) BCL2 inhibitors, (3) immunomodulatory imide drugs (IMIDs) (including lenalidomide and pomalidomide). Participants with autoimmune disease that requires systemic corticosteroids (> 10 mg daily prednisone equivalents) or immunosuppressive medications within 14 days before study drug administration. Inhaled or topical steroids are permitted. NOTE: Steroids for the treatment of other conditions is allowed on study. Prior therapy with an anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CD137, or anti CTLA-4 antibody. Participants who require continuous treatment with a strong CYP3A inhibitor/inducer (i.e., with the exception of any medication to be specifically studied in this protocol).  NOTE: A comprehensive list of inhibitors, inducers, and substrates may be found at: https://drug-interactions.medicine.iu.edu/MainTable.aspx  HIV-positive participants Prior allogeneic stem cell (or other organs) transplant within 6 months or any evidence of active graft-versus-host disease prior to the first dose of study drug Pregnant women- a pregnancy test (urine or serum) with a sensitivity of 25 mIU/mL must be done at screening. Participants with second malignancies requiring active systemic therapy are excluded. Class 3 or 4 congestive heart failure as defined by the New York Heart Association Functional Classification History of any ventricular arrhythmia Unable to swallow capsules, or disease significantly affecting gastrointestinal function, or resection of the stomach or small bowel, or symptomatic inflammatory bowel disease or ulcerative colitis, or partial or complete bowel obstruction. Uncontrolled ongoing or active infection or an infection requiring systemic antibiotics. Concomitant use of warfarin or other vitamin K antagonists Known bleeding disorders (e.g., von Willebrand s disease) or hemophilia. Currently active, clinically significant hepatic impairment (>= moderate hepatic impairment according to the NCI/Child Pugh classification Vaccinated with live, attenuated vaccines within 28 days of first dose of study drug Uncontrolled intercurrent illness or psychiatric illness/social situations that would limit compliance with study requirements.