By Heide Brandes

OKLAHOMA CITY (Reuters) - An Oklahoma man has pleaded guilty to first-degree manslaughter for asphyxiating his stepfather with the underwear he was wearing in a move police dubbed an "atomic wedgie," court officials said on Tuesday.

Brad Davis, 34, who pleaded guilty on Monday, faces between four to 35 years in prison for the death of Denver St. Clair, 58, in December 2013 in McLoud, east of Oklahoma City. A mitigation hearing is set for Wednesday with sentencing scheduled for July.

Davis pulled St. Clair's underwear over his head and around his neck, where the elastic band left ligature marks, police said. The cause of death was blunt force trauma and asphyxiation, Oklahoma Medical Examiner Amy Elliott said.

The two men had been drinking the night of the incident, which took place at the older man's residence. They got into a fight when Davis claimed St. Clair began speaking ill about Davis' mother, who is St. Clair's wife, Pottawatomie County Sheriff Deputy Travis Palmer has said.

Davis never intended to kill St. Clair, his attorney Bob Wyatt told local media.

"He was just tired of taking it. He had been bullied all his life by this guy," Wyatt told The Oklahoman newspaper.

(Writing by Jon Herskovitz; Editing by Lisa Lambert)