Acute myelogenous leukemia (AML) is a hematologic disorder that is characterized by an abnormal proliferation of immature myeloid cells. Granulocytic sarcomas are clusters of leukemic myeloid cells that may develop as a result of AML. Oral manifestations of AML are common and often involve enlargements of the gingiva and/or mucosal tissue from direct leukemia cell infiltration. We describe the case history of a 50-year-old man who had an ulcerative lesion of the oral mucosa that was determined to be a granulocytic sarcoma of AML-MO subtype. The combination of both the subtype and clinical presentation of the leukemia makes this presentation unusual, and to the best of our knowledge, of a type that has not been previously reported in the literature.