Inclusion Criteria:

          -  Participants must have unresectable or metastatic solid malignancy that is
             histologically or cytologically confirmed to be one of the tumor types listed below:

               -  CRC

               -  Gastric adenocarcinoma

               -  GEJ adenocarcinoma

               -  Esophageal adenocarcinoma

               -  Cholangiocarcinoma

               -  Gallbladder carcinoma

          -  Participants must be candidates to receive an oxaliplatin-based regimen as part of
             their standard-of-care treatment.

          -  Participants in phase 1b cohorts or in Cohort 2B can be receiving an oxaliplatin-based
             regimen

               -  For phase 1b cohorts utilizing FOLFOX: up to 2 cycles of FOLFOX (≤85 mg/m2
                  oxaliplatin per 2-week cycle) may have been received prior to Cycle 1 Day 1 of
                  study treatment

               -  For phase 1b cohorts utilizing CAPOX: up to 2 cycles of FOLFOX (≤85 mg/m2
                  oxaliplatin per 2-week cycle) or one cycle of CAPOX (≤130 mg/m2 oxaliplatin per
                  3-week cycle) may have been received prior to Cycle 1 Day 1 of study treatment.

               -  For all phase 1b cohorts: if subject has received oxaliplatin in prior cycles at
                  higher doses than those listed above, there must be a minimum of 28 days off
                  treatment prior to Cycle 1 Day 1 of treatment in this study.

               -  For Cohort 2B prior to the first dose of study treatment:

                    -  Subjects may have received up to 1 cycle of FOLFOX (≤85 mg/m2 oxaliplatin
                       per 2-week cycle) prior to Cycle 1 Day 1 but may not have received prior
                       oxaliplatin for metastatic disease

                    -  Oxaliplatin received in an adjuvant setting is permitted if >6 months prior
                       to Cycle 1 Day 1

                    -  At least 21 days must have elapsed from prior systemic anticancer therapy
                       (including hormonal and biologic therapy but excluding 1 cycle of FOLFOX),
                       non-central nervous system radiation, and treatment with other experimental
                       agents

          -  HER2+ disease, as determined by laboratory testing based on one of the following:

               -  For CRC, cholangiocarcinoma, and gallbladder carcinoma:

                    -  HER2 amplification or overexpression from fresh or archival tumor tissue
                       utilizing one of the following Clinical Laboratory Improvement Amendments
                       (CLIA) certified or International Organization for Standardization (ISO)
                       tests:

                         -  HER2 overexpression (3+ immunohistochemistry [IHC])

                         -  HER2 (ERBB2) amplification by in situ hybridization assay (fluorescence
                            in situ hybridization [FISH] or chromogenic in situ hybridization
                            signal ratio ≥2.0 or gene copy number >6)

                         -  HER2 (ERBB2) amplification by next generation sequencing (NGS) assay

                    -  HER2 amplification in a CLIA certified blood-based NGS assay

               -  Gastric, GEJ, and esophageal adenocarcinomas must use the following criteria:

                    -  HER2+ overexpression (IHC3+) by an FDA approved assay, from a newly obtained
                       biopsy or surgical specimen, evaluated following the package insert's
                       interpretational manual for gastric adenocarcinoma. IHC2+ is eligible if the
                       tumor is HER2 amplified by an FDA approved in situ hybridization assay

          -  Phase 1b cohorts: measurable or non-measurable disease according to RECIST v1.1 as
             determined by the investigator

          -  Phase 2 cohorts: measurable disease according to RECIST v1.1 as determined by the
             investigator

          -  Eastern Cooperative Oncology Group Performance Status score of 0 or 1.

        Exclusion Criteria:

          -  History of known hypersensitivity to oxaliplatin, fluoropyrimidines, leucovorin,
             trastuzumab, or compounds chemically or biologically similar to tucatinib, except for
             Grade 1 or 2 infusion related reactions to oxaliplatin or trastuzumab that were
             successfully managed, or known allergy to any of the excipients in the study drugs

          -  Treatment with oxaliplatin dose in excess of the limitations specified in the
             inclusion criteria

        There are additional inclusion criteria. The study center will determine if criteria for
        participations are met.