SAN DIEGO -- Actor Ryan O'Neal's oldest son, who was on drugs when his car veered into oncoming traffic on San Pasqual Valley Road and collided head-on with a van, pleaded guilty Tuesday to DUI causing injury and being a felon in possession of a gun.

Griffin O'Neal, 47, faces a maximum term of four years in prison, but could also get a low-end probationary sentence, when he goes back before Superior Court Judge Eugenia Eyherabide on Jan. 12.

Charges of possession of rock cocaine and drug paraphernalia were dismissed as a result of the plea, in which O'Neal also admitted a 1992 conviction for shooting into his ex-girlfriend's unoccupied vehicle, said Deputy District Attorney Vanessa DuVall.

The defendant was on probation for a 2007 DUI conviction when the accident occurred last Aug. 2 about 1 p.m. near San Diego Safari Park. He was arrested a month later.

DuVall said Griffin was charged after toxicology tests showed he had amphetamines, cocaine, marijuana and Xanax in his system at the time of the crash. She said a loaded firearm, a shotgun and ammunition for both were found in O'Neal's car.

Both O'Neal and the other driver were taken to a hospital for treatment of injuries from the crash, DuVall said.

Defense attorney Heather Boxeth told a judge at O'Neal's arraignment that her client -- who has a wife and 4-year-old son and lives in North County - - was trying to come to the aid of his 26-year-old half-brother when he got behind the wheel the day of the accident. She did not elaborate.

Redmond O'Neal, son of Ryan O'Neal and the late Farrah Fawcett, was arrested in Santa Monica the same day, after police found heroin in his car during a traffic stop. He has a lengthy history of drug problems.

Griffin O'Neal appeared in several films in his younger years, including "Escape Artist,'' "April Fool's Day'' and "Assault of the Killer Bimbos.'' He drew an 18-day jail sentence in 1987 for failing to perform community service after he was convicted of reckless boating in an accident that killed the son of director Francis Ford Coppola.