Inclusion Criteria for All Cohorts
- At least one bi-dimensionally measurable nodal lesion, defined as > 1.5 cm in its longest dimension, or one bi-dimensionally measurable extranodal lesion, defined as > 1.0 cm in its longest diameter
- Adequate hematologic function
- Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2; with the exception of South Korea, where participants 80 years or older with ECOG >/= 2 will not be eligible
Inclusion Criteria Specific to Cohort A
Participants in Cohort A must also meet the following criteria for study entry:
- Histologically confirmed DLBCL according to World Health Organization (WHO) 2016 expected to express the cluster of differentiation-20 (CD20) antigen
- One prior therapy with any systemic anthracycline-based chemoimmunotherapy containing regimen for previously untreated DLBCL
- Best response of SD or PR to prior systemic chemoimmunotherapy at the end of induction treatment in accordance with the Lugano 2014 criteria
Inclusion Criteria Specific to Cohorts B and C
Participants in Cohorts B and C must also meet the following criteria for study entry:
- Previously untreated, histologically confirmed, DLBCL according to WHO 2016 classification
- Age >/= 80 years, or
- Age 65-79 years and considered ineligible for chemoimmuotherapy (R-CHOP) with at least one of the following: Impairment in at least two activity of daily living (ADL) components as defined in the protocol; impairment in at least two instrumental ADL components as defined in the protocol; cumulative illness rating scale - geriactic (CIRS-G) score of at least one cormorbidity with a severity score of 3-4 (not including lymphoma and hematologic deficiencies due to lymphoma) or a score of 2 in >/= 8 comorbidities; impairment in cardiac function, renal function, liver function, or other comorbidities such that the participant is unfit for full-dose immunochemotherapy, such as rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP)
- Participants with an initial ECOG performance status of 3 may be considered during screening if the performance status is DLBCL-related and if pre-phase treatment during the screening phase (not more than 100 mg/day up to 7 days prior to Cycle 1 Day 1) results in an improvement of ECOG performance status to </= 2 prior to enrollment
Exclusion Criteria for All Cohorts
Participants who meet any of the following criteria will be excluded from study entry:
- Transformed lymphoma
- CNS lymphoma
- Prior treatment with mosunetuzumab
- Prior stem cell transplant (autologous and allogeneic)
- History of confirmed progressive multifocal leukoencephalopathy (PML)
- Known or suspected chronic active Epstein Barr virus (CAEBV), hepatitis B, hepatitis C (HCV), or Human Immunodeficiency Virus (HIV)
- Known or suspected history of hemophagocytic lymphohistiocytosis (HLH)
- Positive SARS-CoV-2 antigen or PCR test within 30 days prior to Cycle 1 Day 1
- Prior solid organ transplantation
- Current or past history of central nervous system (CNS) disease, such as stroke, epilepsy, CNS vasculitis, or neurodegenerative disease
- Clinically significant history of liver disease
- Prior treatment with radiotherapy within 2 weeks prior to Cycle 1, Day 1 (C1D1)
- Significant cardiovascular disease
Exclusion Criteria Specific to Cohort A
Participants in Cohort A who meet the following criteria will be excluded from study entry:
- Prior anti-lymphoma treatment with chemotherapy, immunotherapy, or biologic therapy 4 weeks prior to C1D1
Exclusion Criterion Specific to Cohorts B and C
Participants in Cohorts B and C who meet the following criterion will be excluded from study entry:
- Prior treatment for DLBCL with chemotherapy, immunotherapy, and biologic therapy
Exclusion Criteria Specific to Cohort C
Participants in Cohort C who meet the following criteria will be excluded from study entry:
- Current Grade >1 peripheral neuropathy by clinical examination or demyelinating form of Charcot-Marie-Tooth disease