Earlier, O'Neal was formally charged with possession of a controlled substance and possession of a firearm by a felon.
Speaking outside the court, his famous father said 26-year-old O'Neal had struggled since the death of Fawcett.
"He never recovered from the loss of his mother," Ryan O'Neal told People magazine.
Redmond's half-sister Tatum O'Neal, who also attended the court session, added "He's an addict. Addicts keep using until they want to stop. We're praying for him every day."
Police in Santa Monica, Calif., pulled O'Neal over after they saw his vehicle run a red light about 8:15am local time Tuesday. When officers searched the vehicle, they found an unspecified quantity of heroin, police allege.
Officers also allegedly found marijuana in the vehicle and after a search of his apartment recovered a loaded handgun.
In court Thursday, O'Neal was ordered held without bail until his next court appearance on Aug. 24.
O'Neal completed a court-ordered drug rehabilitation program in August 2010 after being arrested in December 2009 on drug charges while on a 24-hour pass from a California rehab facility.
He was previously arrested on drug smuggling charges in April 2009.
Fawcett died on June 25, 2009. O'Neal was in Los Angeles County Jail at the time, but was permitted to leave to attend her funeral.