Inclusion Criteria:

          1. Patients aged ≥18 years at time of screening.

          2. Written informed consent by the patient (or their legal representative) prior to
             admission to this study. In addition, any locally required authorization (Health
             Insurance Portability and Accountability Act in the US), must be obtained from the
             patient prior to performing any protocol-related procedures, including screening
             evaluations.

          3. Adequate renal and hepatic function defined as:

               1. Total bilirubin within 1.5 x upper limit of normal (ULN), except those with
                  Gilberts syndrome for whom this must be ≤3 x ULN

               2. AST and ALT ≤2.5 x ULN

               3. Calculated creatinine clearance ≥45 mL/min

               4. Serum albumin ≥2.5 g/dL For any patient with laboratory values outside the ranges
                  outlined above that are considered due to the patient's underlying disease (AML
                  or MDS), the patient may be enrolled into the study following consultation
                  between the Investigator and the Sponsor's Medical Officer, if the patient is
                  likely to benefit from receiving ONO-7475 (based on the Investigator's
                  assessment).

          4. Eastern Cooperative Oncology Group (ECOG) performance status 0-2 as assessed during
             the screening period and then again anytime during the 2-day period immediately
             preceding the start of dosing in Parts A and D.

          5. Life expectancy of at least 3 months

          6. Sexually active female patients of childbearing potential and sexually active male
             patients must agree to use an effective method of birth control (e.g., barrier methods
             with spermicides, oral or parenteral contraceptives and/or intrauterine devices)
             during the entire duration of the study and for 4 months after final administration of
             study drug. Note that sterility in female patients must be confirmed in the patients'
             medical records and be defined as any of the following: surgical hysterectomy with
             bilateral oophorectomy, bilateral tubular ligation, natural menopause with last menses
             >1 year ago, radiation-induced oophorectomy with last menses >1 year ago,
             chemotherapy-induced menopause with last menses >1 year ago.

          7. Diagnosis of AML or MDS according to WHO criteria 2016 (Part A only).

          8. Either criterion is met (Part A only):

               1. Patients with R/R AML with at least 5% blasts by BM biopsy or aspirate, or at
                  least 1% blasts in peripheral blood, not likely to benefit from standard salvage
                  chemotherapy

               2. Patients with R/R MDS who are either not eligible for (or unlikely to benefit
                  from) other forms of therapy, including HSCT, according to the treating
                  Physician/Investigator .

          9. All patients must have received at least one previous line of therapy (Part A only).

         10. Diagnosis of AML according to WHO criteria (2016) (Part D only).

         11. Patients with R/R AML who have no standard-of-care options known to provide clinical
             benefit in patients with R/R AML (Part D only)

               1. Refractory AML: Patients who have not achieved complete remission after two
                  cycles of induction chemotherapy (i.e., anthracycline containing regimen), four
                  cycles of hypomethylating agents, or two cycles of other AML therapy

               2. Relapsed AML: Patients who have ≥5% BM blasts in BM, or reappearance of blasts in
                  the peripheral blood not attributable to another cause (e.g., recovery of normal
                  cells following chemotherapy-induced aplasia) or (re)appearance of extramedullary
                  disease after CR of prior AML therapy.

         12. All patients must have measured BM aspirate blast counts at Screening. Where the
             aspirate is hypo cellular or inaspirable a biopsy would be considered (Part D only).

         13. All patients must have received 1 to 3 (inclusive) prior AML therapies after first
             diagnosis of AML (Part D).

        Exclusion Criteria:

          1. Patients with active central nervous system leukemia.

          2. QT interval corrected according to Fredericia's formula (QTcF) prolongation defined as
             a QTcF interval >470 msec or other significant ECG abnormalities including second
             degree (type II) or third degree atrioventricular block or bradycardia (ventricular
             rate <50 beats/min).

          3. Clinically significant liver disease, including active viral or other hepatitis,
             current alcohol abuse, or severe cirrhosis.

          4. Human immunodeficiency virus (HIV), active hepatitis B (HBV) or C (HCV) infection.

          5. Retinal disease (e.g., retinitis pigmentosa including Mertk mutations), retinal
             hemorrhage or any disorder which may inhibit follow up for retinal toxicity.

          6. Serious intercurrent medical or psychiatric illness that will prevent participation or
             compliance with study procedures, including serious active infection (including
             COVID-19).

          7. Acute promyelocytic leukemia (the French-American-British M3 classification).

          8. Patients not recovered to Grade 1 or stabilized from the effects (excluding alopecia)
             of any prior therapy for their malignancies.

          9. Concurrent treatment with other investigational drugs.

         10. Daily requirement of ≥10 mg/day of prednisone or equivalent dose of other
             corticosteroids.

         11. Prior HSCT within 12 weeks of the first dose of study treatment or ongoing
             immunosuppressive therapy for graft-versus-host disease.

         12. Participation in another clinical trial with any investigational drug within 14 days
             or with any licensed drug within five half-lives, prior to the first ONO-7475 dosing
             (for Part A) or prior to the first venetoclax dosing (for Part D).

         13. Prior AML or MDS therapy (non-experimental) within 14 days or 5 half-lives, whichever
             is longer, prior to the first dose of ONO-7475 (for Part A) or prior to the first
             venetoclax dosing (for Part D) (except those permitted in Section 7.1) and no residual
             toxicity from the prior therapy hindering of the ONO-7475 dosing (for Part A) or
             ONO-7475 plus venetoclax dosing (for Part D).

         14. Prior radiotherapy within 21 days of screening, with the exception of localized
             palliative radiotherapy.

         15. Patients undergoing current treatments for other cancers.

         16. Pregnant or lactating women.

         17. Proliferative disease (white blood cell [WBC] counts >30 x 10e9/L) confirmed prior to
             the first dose of ONO-7475 (for Part A) or WBC >25 x 10e9/L in Part D.

         18. Active malignancy, other than AML (Parts A and D) or MDS (Part A), requiring systemic
             therapy except for those patients who have been diagnosed with either prostate or
             breast cancer and who have received a stable dose of hormone therapy for a minimum of
             6 months prior to entering this study.

         19. Known hypersensitivity to venetoclax (Part D only).