The mountainous and forested area that now makes up Oddar Meanchey Province was part of Siamese province of Monthon Isan from 1795 to 1907 under the supervision of Sangkha. After the neighboring Thai province of Inner Cambodia, and Oddar Meanchey, was returned to Cambodia in 1907, King Sisowath split the province into Battambang and Siem Reap (Oddar Meanchey was included with Siem Reap). Following the Franco-Thai War from 1941 to 1946 the province was under Siamese administration as Phibunsongkhram Province. The province was returned to Cambodian rule in 1946 following French diplomatic efforts at the end of World War II. The province was officially organized in 1966. During the Cambodian Civil War, the remote area was used as a base by the Khmer Rouge when they fought against the Khmer Republic led by general Lon Nol.