Oscar S. Heizer was an American diplomat who served in various posts as Consul General in the Ottoman Empire. Heizer, who was the Consul General in Trebizond during World War I, witnessed the Armenian genocide and often risked his own life to save the lives of Armenians.

Trebizond was a city in the Ottoman Empire where the Armenian genocide occurred. The method employed to kill was mainly by mass drowning, resulting in estimated deaths of 50,000 Armenians. The city was also an important location of subsequent trials held to prosecute those involved with the systematic massacre. Cemal Azmi, the governor of Trebizond during the genocide, was later assassinated as part of Operation Nemesis.