Testing the Addition of a New Anti-cancer Drug, Venetoclax, to the Usual Treatment (Ibrutinib and Obinutuzumab) in Untreated, Older Patients With Chronic Lymphocytic Leukemia

Inclusion Criteria: * PRE-REGISTRATION ELIGIBILITY CRITERIA (STEP 0) * Patients must have been diagnosed with chronic lymphocytic leukemia (CLL) (\> 5000 B-cells per uL of peripheral blood at any point during the course of their disease) or small lymphocytic lymphoma (SLL) with \< 5000 B-cells per uL of blood but with disease-associated lymphadenopathy * This blood submission is mandatory prior to registration/randomization to perform fluorescence in situ hybridization (FISH) centrally that will be used for stratification. It should be obtained as soon after pre-registration as possible * REGISTRATION ELIGIBILITY CRITERIA (STEP 1) * Patients must be diagnosed with CLL or SLL in accordance with 2018 International Workshop on Chronic Lymphocytic Leukemia (IWCLL) criteria that includes all of the following: * \>= 5 x10\^9 B lymphocytes (5000/uL) in the peripheral blood measured by flow cytometry at any point in the course of the disease or less peripheral blood involvement but disease-associated lymphadenopathy * On local morphologic review, the leukemic cells must be small mature lymphocytes, and prolymphocytes must not exceed 55% of the blood lymphocytes * Neoplastic cells on immunophenotype (performed locally) must reveal a clonal B-cell population, which express the B cell surface markers of CD19 and CD20, as well as the T-cell antigen CD5. Patients with bright surface immunoglobulin expression or lack of CD23 expression in \>10% of cells must lack t(11;14) transloca…