A Study of JNJ-74856665 in Participants With Acute Myeloid Leukemia (AML) or Myelodysplastic Syndrome (MDS)

Inclusion Criteria:

* A diagnosis of: Arms A and C: Acute Myeloid Leukemia (AML) according to the World Health Organization (WHO) 2016 criteria with relapsed or refractory disease and have exhausted or are ineligible for standard therapeutic options; or newly transformed secondary AML according to the WHO 2016 criteria and have exhausted standard therapeutic options for AML during their treatment prior to transformation; or high-risk or very high-risk Myelodysplastic Syndrome (MDS) according to the WHO 2016 criteria and the revised International Prognostic Scoring System (IPSS-R) with relapsed or refractory disease and have exhausted or are ineligible for standard therapeutic options; or Arm A only: chronic myelomonocytic leukemia-2 (CMML-2) according to the WHO 2016 criteria with relapsed or refractory disease and have exhausted or are ineligible for standard therapeutic options; Arm B: Eligible participants must be considered unsuitable for intensive treatment with a curative intent (including stem cell transplantation), but eligible to receive Azacitidine (AZA) treatment with the following underlying diseases: AML (newly diagnosed or relapsed/refractory) according to the 2016 WHO classification only if Venetoclax (VEN) + hypomethylating agent (HMA) (or low-dose cytarabine) is not indicated or available; or high-risk or very high-risk MDS according to the 2016 WHO classification and IPSS-R; or CMML-2 according to the WHO 2016 criteria; Arm D: Very low, low, or intermediate-risk MDS according to the 2016 WHO classification and IPSS-R and the following: Transfusion dependence defined as requiring at least 3 red blood cell (RBC) units transfused within 16 weeks prior to C1D1; pre-transfusion hemoglobin (Hb) should be less than (\<) 9.0 grams per decilitre (g/dL) to count towards the 3 units total, Relapsed/refractory to erythropoiesis-stimulating agent (ESA) treatment or endogenous serum erythropoietin (EPO) level greater than (\>) 500 milliunits per milliliter (mU/mL). Exception: Del(5q) karyotype is allowed, provided prior treatment with lenalidomide has failed or participant was ineligible to receive lenalidomide
* Eastern Cooperative Oncology Group (ECOG) performance status grade of 0 or 1
* Women of childbearing potential (WOCBP) must have a negative highly sensitive serum (beta-human chorionic gonadotropin) at screening and again within 48 hours prior to the first dose of study treatment. A urine or serum test is acceptable at subsequent time points
* A WOCBP must agree to all the following during the study and for 6 months after the last dose of study treatment: a) use a barrier method of contraception; b) use a highly effective preferably user-independent method of contraception; c) not to donate eggs (ova, oocytes) or freeze for future use for the purposes of assisted reproduction; d) not plan to become pregnant; e) not breast-feeding
* A male must agree to all the following during the study and for 90 days after the last dose of study treatment: a) wear a condom when engaging in any activity that allows for passage of ejaculate to another person; b) not to donate sperm or freeze for future use for the purpose of reproduction; c) not plan to father a child. In addition, the participant should be advised of the benefit for a female partner to use a highly effective method of contraception as condom may break or leak

Exclusion Criteria:

* Acute promyelocytic leukemia according to World Health Organization 2016 criteria
* Known central nervous system involvement
* Prior treatment with a dihydroorotate dehydrogenase (DHODH) inhibitor for an oncology indication or intolerance to a DHODH inhibitor given for non-oncology indication
* Toxicities (except for alopecia, peripheral neuropathy, thrombocytopenia, neutropenia, anemia) from previous anticancer therapies that have not resolved to baseline or to Grade 1 or less
* Known allergies, hypersensitivity, or intolerance to JNJ-74856665, AZA, or VEN or the excipients of these treatments