O'Neal, 24, appeared Tuesday at a brief hearing before Superior Court Judge Michael Tynan after being jailed on Dec. 29.
O'Neal was released from jail last year after a series of drug-related arrests and told to attend an intense drug treatment program that would be his last chance to avoid up to six years in state prison.
It was unclear what prompted Tynan to find O'Neal in violation of his probation, but the program required O'Neal to undergo frequent drug testing.
District attorney's spokeswoman Jane Robison said O'Neal gave Tynan a sealed, handwritten letter during Tuesday's hearing.
O'Neal's attorney Richard Pintal declined to discuss its contents, saying Tynan told O'Neal to write the letter as an assignment.
O'Neal will remain in custody until he returns to court on Feb. 2, Robison said.
Ryan O'Neal was not present in court Tuesday.
Redmond O'Neal was in jail during Fawcett's final days. He was briefly released for a last visit and allowed to attend her funeral.