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: * DLBCL, Not Otherwise Specified (NOS). * High grade B-cell lymphoma with MYC and BCL-2 and/or BCL-6 rearrangement with DLBCL morphology. * T-cell/histiocyte-rich large B-cell lymphoma. * Epstein Barr virus-positive DLBCL, NOS. * 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. * Availability of archival or fresh or paraffin embedded tissue at Screening. * 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. * 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 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 * Positron emission tomography (PET)-positive on PET-CT scan. * Laboratory values meeting the criteria laid out in the protocol. * Left ventricular ejection fraction must be \>= 50% by multi-gated acquisition or transthoracic echocardiography at Screening. Exclusion Criteria: * 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. * Clinically significant cardiovascular disease as per the protocol.