A Study to Evaluate Change in Disease Activity of Subcutaneous (SC) Epcoritamab Combined With Intravenous and Oral Rituximab, Cyclophosphamide, Doxorubicin Hydrochloride, Vincristine, and Prednisone (R-CHOP) or R-CHOP in Adult Participants With Newly Diagnosed Diffuse Large B-Cell Lymphoma (DLBCL)

Inclusion Criteria:

Planned to receive treatment with 6 cycles of standard rituximab, cyclophosphamide, doxorubicin hydrochloride, vincristine, and prednisone (R-CHOP) per investigator determination.

Must have newly diagnosed, histologically confirmed CD20+ diffuse large b-cell lymphoma [DLBCL] (de novo or histologically transformed from a diagnosis of follicular lymphoma) at most recent representative tumor biopsy based on the pathology report, with a World Health Organization (WHO) 2016 classification and including:

1. DLBCL, Not Otherwise Specified (NOS).
2. High grade B-cell lymphoma with MYC and BCL-2 and/or BCL-6 rearrangement with DLBCL morphology.
3. T-cell/histiocyte-rich large B-cell lymphoma.
4. Epstein Barr virus-positive DLBCL, NOS.
5. Follicular lymphoma Grade 3b.

Note: The local pathology report must be available at Screening to support CD20+ DLBCL histology.

Composite/intermediate histology with any of the following components is not allowed: high grade B-cell lymphoma, NOS; Hodgkin's lymphoma; primary mediastinal (thymic) large B-cell lymphoma; Burkitt; plasmablastic lymphoma or any CD20- lymphoma, such as anaplastic lymphoma kinase-positive large B-cell lymphoma, human herpesvirus type 8-positive DLBCL, or primary effusion lymphoma.

1. Availability of archival or freshly collected tumor tissue at Screening. Archival paraffin-embedded tissue must be obtained within 8 weeks prior to Cycle 1 Day 1.
2. Must have an IPI score of 2-5. The number of participants with IPI 2 will not exceed approximately 30% of the overall sample size.
3. Must have an Eastern Cooperative Oncology Group (ECOG) performance status score of 0-2 prior to initiating R-CHOP treatment. Note that participant with an initial ECOG performance status >= 3 may be screened if pre-phase treatment is planned. Participant may be eligible if ECOG performance status were to improve to 0-2 during pre-phase treatment.

Has at least one target lesion defined as:

1. >= 1 measurable nodal lesion (long axis > 1.5 cm ) or >= 1 measurable extra-nodal lesion (long axis > 1 cm) on computed tomography (CT) scan or magnetic resonance imaging (MRI). AND
2. Positron emission tomography (PET)-positive on PET-CT scan.
3. Laboratory values meeting the criteria laid out in the protocol.
4. Left ventricular ejection fraction must be >= 50% by multi-gated acquisition or transthoracic echocardiography at Screening.

Exclusion Criteria:

1. History of prior systemic anti-lymphoma therapy for diagnosed diffuse large b-cell lymphoma (DLBCL) including any definitive radiotherapy with curative intent] other than corticosteroids with or without vincristine during prephase treatment, or non-curative intent palliative radiotherapy with the stipulation that radiated lesions cannot be selected as target lesion for response assessment.
2. Clinically significant cardiovascular disease as per the protocol.