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

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 chemotherapy, and infusion will be counted as one line of therapy.

Local therapies (e.g., radiotherapy) will not be considered as lines of treatment

For participants with a history of the transformation of indolent disease to DLBCL, it is preferred that participants have received at least one treatment for the transformed lymphoma. However, if there are cases where the participants have received an anthracycline-containing chemotherapy regimen (such as R-CHOP) for the indolent lymphoma only, then these participants can be considered as eligible.

At least one bi-dimensionally measurable lesion, defined as \> 1.5 cm in its longest dimension as measured by CT or MRI

Eastern Cooperative Oncology Group (ECOG) performance status of 0,1 or 2

Adequate hematological function

For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraception, and agreement to refrain from donating eggs

For men: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures, and agreement to refrain from donating sperm,

What this study is about