The charges stem from a September 17 incident, when Redmond, 24, and Ryan, 67, were arrested at the actor's home in Malibu.
Redmond, whose mother is Farrah Fawcett, entered the plea against one felony count of methamphetamine possession, two misdemeanor counts of possession of drug paraphernalia and one misdemeanor count of illegal possession of pepper spray.
His next court date is scheduled for May 14.
As previously reported on AccessHollywood.com, authorities arrested both men in September on suspicion of narcotics possession after discovering methamphetamine during a probation search at the senior O'Neal's home, where Redmond O'Neal resides.
Redmond O'Neal was previously arrested on drugs charges and driving under the influence stemming from an incident in January 2008 when he was pulled over in Malibu.
The younger O'Neal was charged eventually charged with possession of heroin and methamphetamine and driving under the influence.
In June 2008, he pleaded guilty to possession and a misdemeanor DUI charge and managed to avoid jail, but was sentenced to three years probation.
In October 2008, a judge ordered Redmond to a rehab and detox facility after he admitted he had relapsed on drugs.
Related Content from AccessHollywood.com:
PLAY IT NOW: Redmond O'Neal Spotted In Malibu (September 17, 2008)
PLAY IT NOW: Access Extended: Tatum O'Neal