A Study Evaluating Safety and Efficacy of Venetoclax in Combination With Azacitidine Versus Standard of Care After Allogeneic Stem Cell Transplantation (SCT) in Participants With Acute Myeloid Leukemia (AML)

* Participants must be at least 18 years old for Part 1 and, at least 12 years old for Part 2. * Participant must be diagnosed with Acute Myeloid Leukemia (AML) by World Health Organization (WHO) criteria (2017) and either be planning for allogeneic stem cell transplantation or have received allogeneic stem cell transplantation within the past 60 days. * Blast percentage in bone marrow before transplant must be \< 10%. * Blast count in peripheral blood must be "0" and Blast percentage in bone marrow must be \< 5% after transplant. * Participant meet adequate renal, hepatic and hematologic criteria as described in the protocol. * Participants \>= 17 years old must have a Karnofsky Performance Scale (KPS) score \> 50 and participants between 12 to 16 years old must have a Lansky Play Performance Scale score \> 40.

* History of disease progression during prior treatment with venetoclax. * History of any other malignancy within 2 years prior to study entry, except for: Adequately treated in situ carcinoma of the cervix uteri or carcinoma in situ of breast; basal cell carcinoma of the skin or localized squamous cell carcinoma of the skin; previous malignancy confined and surgically resected (or treated with other modalities) with curative intent; Myelodysplastic Syndrome, Myeloproliferative neoplasm (only allowed if it transformed to AML and AML should be the indication for marrow transplantation). * Participant has known infection with HIV or history of being positive for hepatitis B virus (HBV) or hepatitis C virus (HCV) infection. * Presence of clinical or laboratory symptoms/signs of extramedullary myeloid malignancy.