Key Inclusion Criteria:

1. Male or female participants ≥ 55 years of age
2. Newly diagnosed, histologically confirmed de novo acute myeloid leukemia (AML) or AML secondary to prior myelodysplastic disease or CMML (Chronic myelomonocytic leukemia)
3. First complete remission (CR)/ complete remission with incomplete blood count recovery (CRi) with induction therapy with intensive chemotherapy with or without consolidation therapy within 4 months (+/- 7 days of achieving CR or CRi)
4. Eastern Cooperative Oncology Group (ECOG) performance status - 0, 1, 2, 3

Key Inclusion Criteria in the Extended Phase of the study:

At the Investigator's discretion and with approval of the sponsor, participants meeting all of the following eligibility criteria are eligible to enter the extension phase:

1. All participants randomized into the oral azacitidine or placebo arm and are continuing in either the treatment phase or follow-up phase of the CC-486-AML-001 study;

* Participants randomized to oral azacitidine treatment arm and continuing in the treatment phase demonstrating clinical benefit as assessed by the investigator are eligible to receive oral azacitidine in the extension phase (EP);
* Participants randomized into placebo arm of the study will not receive oral azacitidine in the EP, but will be followed for survival in the EP;
* Participants currently in the follow-up phase will continue to be followed for survival in the EP;
2. Participants who have signed the informed consent for the EP of the study;
3. Participants who do not meet any of the criteria for study discontinuation

Key Exclusion Criteria:

1. AML with inversion (inv)(16), translocation = t(8;21), t(16;16), t(15;17), or t(9;22) or molecular evidence of such translocations
2. Prior bone marrow or stem cell transplantation
3. Have achieved CR/CRi following therapy with hypomethylating agents
4. Diagnosis of malignant disease within the previous 12 months
5. Proven central nervous system (CNS) leukemia