The actor - whose partner Farrah Fawcett died from cancer in 2009 - released a statement to People saying he's hopeful that he will overcome the disease.
"Although I was shocked and stunned by the news, I feel fortunate that it was detected early and according to my extraordinary team of doctors the prognosis is positive for a full recovery," O'Neal revealed.
The star's press statement continued: "I am deeply grateful for the support of my friends and family during this time, and I urge everyone to get regular check-ups, as early detection is the best defence against this horrible disease that has afflicted so many."
O'Neal previously battled myelogenous leukemia in 2001, but went into remission five years later.
The actor most recently starred with daughter Tatum O'Neal in the reality series The O'Neals, which documented their attempts to heal their famously-troubled relationship.
Ryan later admitted that he only reconciled with Tatum for the series, but they never truly resolved their differences.
> Tatum O'Neal denies drug relapse
Watch Ryan O'Neal in a trailer for Paper Moon below: