Redmond's mother and O'Neal's longtime partner, actress Farrah Fawcett, died of cancer at the age of 62.
Redmond, 26, was on probation for prior drug crimes when he was arrested again after heroin and marijuana were allegedly found in his vehicle during a traffic stop. A loaded handgun allegedly was discovered at his apartment after a subsequent search warrant was executed by police.
Redmond pleaded not guilty Thursday in a Los Angeles court to felony possession of heroin and being a felon in possession of a handgun.
"He never recovered from the loss of his mother," Ryan O'Neal told People.com outside the Los Angeles court where his son entered his plea.
"He's an addict," said Redmond's half-sister, actress Tatum O'Neal. "Addicts keep using until they want to stop. We're praying for him every day."
Redmond is being held without bail until his Aug. 24 court date.