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 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; chemotherapy 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; chemotherapy followed by
             allogeneic HSCT will be counted as one line of therapy

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

          -  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,

        Exclusion Criteria:

          -  History of severe allergic or anaphylactic reactions to humanized or murine monoclonal
             antibodies (or recombinant antibody-related fusion proteins) or known sensitivity or
             allergy to murine products

          -  Contraindication to rituximab, gemcitabine or oxaliplatin

          -  Peripheral neuropathy assessed to be > Grade 1 according to NCI CTCAE v5.0

          -  Prior use of polatuzumab vedotin or a gemcitabine plus platinum-based agent
             combination, recent participation in a clinical trial, and/or treatment with
             radiotherapy, chemotherapy, immunotherapy, immunosuppressive therapy within 2 weeks

          -  Planned autologous or allogenic stem cell transplantation at time of recruitment

          -  Primary or secondary central nervous system (CNS) lymphoma

          -  Richter's transformation or prior CLL

          -  Abnormal laboratory values or health conditions, as assessed by the investigator, any
             known conditions preventing adherence to protocol or active bacterial, viral, fungal,
             mycobacterial, parasitic, or other infection

          -  Vaccination with a live vaccine within 4 weeks prior to treatment

          -  Recent major surgery (within 6 weeks before the start of Cycle 1 Day 1) other than for
             diagnosis

          -  Any other diseases, metabolic dysfunction, physical examination finding, or clinical
             laboratory finding giving reasonable suspicion of a disease or condition that
             contraindicates the use of an investigational drug or that may affect the
             interpretation of the results or render the patient at high risk from treatment
             complications

          -  Pregnant or breastfeeding, or intending to become pregnant during the study or within
             12 months after the last dose of study drug

          -  Women of childbearing potential must have a negative serum pregnancy test result
             within 7 days prior to initiation of study drug