Oler Ammon Bartley Jr., 50, a retired Foreign Service officer who was an authority on Southeast Asia, died of cancer Tuesday at Goerge Washington University.

He became ill while serving as deputy chief of mission in Rangoon, Burma, and returned here in March 1978. He retired this past February.

Mr. Bartley joined the Foreign Service in 1956. His assignments included Penang and Kuala Lumpur in Malaysia, Surabaja, Indonesia, and Ban Me Thuot and Nha Trang in South Vietnam.

His tours of duty at the State Department here were in personnel, on the secretariat staff, as deputy director of the Vietnam desk, as a member of the U.S. delegation to the United Nations and of the board of examiners for the Foreign Service.

Mr. Bartley was born in Salisbury, Md. He was a graduate of Wesley Junior College in Dover, Del., and Drew University in Madison, N.J. He also attended the University of Delaware and the University of Pennsylvania.

He served with the Navy Reserves during the Korean conflict and was a destroyer officer when he was discharged at Newport, R.I. in 1955.

He is survived by his wife, the former Mary Carmen Hurst, a daughter, Lisa F., and two sons, Ammon Hurst and Charles Mareen, all of the home in Vienna, Va.; his parents, the Rev. Dr. and Mrs O. A. Bartley, of Salisbury, N.C., and a sister, Frances Cleaver, of Washington.

The family suggests that expressions of sympathy be in the form of contributions to the American Cancer Society or the American Foreign Service Scholarship Fund.