Marine Sgt. Orlando N. Hamilton, 23, pleaded guilty yesterday to first-degree murder in the strangling of 15-year-old Carolyn Oberman of Upper Marlboro in June 1984.

Hamilton, an instructor at a Naval air base artillery school near San Diego, agreed to plead guilty in Prince George's County Circuit Court after prosecutors said they would not seek the death penalty, his defense attorney, Leonard Stamm, said.

Prosecutors also agreed to drop four kidnaping and armed robbery charges, he said.

Assistant State's Attorney William Shockley said that Hamilton, who will be sentenced March 11, could receive a maximum sentence of life in prison.

The possibility that Hamilton could have received the death penalty was "a very powerful incentive to dispose of the case other than by going to trial," Stamm said.

The body of Oberman, a sophomore at La Reine High School in Suitland, was found by her father in a wooded area near their home in the Marlboro Meadows subdivision the day after she disappeared.

Hamilton, who was on leave from California visiting his family in the subdivision at the time, was arrested in January 1985 in San Diego after Prince George's detectives traveled there to question him.