A Study to Evaluate the Safety and Efficacy of Polatuzumab Vedotin in Combination With Rituximab, Gemcitabine and Oxaliplatin Compared to Rituximab, Gemcitabine and Oxaliplatin Alone in Participants With Relapsed or Refractory Diffuse Large B-Cell Lymphoma

Inclusion Criteria: * Histologically-confirmed diffuse large B-cell lymphoma, not otherwise specified (NOS) or history of transformation of indolent disease to DLBCL * Relapsed disease (disease that has recurred following a response that lasted ≥ 6 months from completion of the last line of therapy) or refractory disease (disease that did not respond to or that progressed during therapy or progressed within 6 months (\< 6 months) of prior therapy) * At least one (≥ 1) line of prior systemic therapy: * Patients may have undergone autologous hematopoietic stem cell transplantation (HSCT) prior to recruitment; In such cases, salvage chemotherapy (e.g., rituximab, dexamethasone, cytarabine, and cisplatin \[R-DHAP\] and rituximab, ifosfamide, carboplatin, and etoposide phosphate \[R-ICE\]) will be counted as one line of therapy and conditioning chemotherapy (e.g., BEAM) followed by consolidative autologous HSCT will be counted as one line of therapy * Patients may have undergone allogeneic HSCT prior to recruitment, so long as they are off all immunosuppressive therapy and have no active GVHD; In such cases, salvage chemotherapy (e.g., R-DHAP and R-ICE) will be counted as one line of therapy and conditioning chemotherapy (e.g., carmustine, etoposide, cytarabine, and melphalan \[BEAM\]) followed by allogeneic HSCT will be counted as a separate line of therapy * Participants may have undergone CAR T-cell therapy prior to recruitment. In such cases, cell collection, conditioning che…