O'Neal said in a statement issued to People that the cancer was found early, and that his doctors have given him a positive prognosis "for a full recovery."
"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 defense against this horrible disease that has afflicted so many," he said in the statement issued to People.
O'Neal has previously battled cancer in his life; he was treated for myelogenous leukemia in the '90s, People reported.
O'Neal's longtime partner, actress Farrah Fawcett, had also battled cancer, ultimately passing away in 2009 from the disease, the Associated Press reported.
Prostate cancer is put in terms of stages, and with the lower the stage, the less dangerous it is. The National Cancer Institute reported that the stages also correlate to whether the cancer has spread outside of the prostate.
According to the National Cancer Institute, the stages are also defined by a man's Gleason score and PSA (prostate-specific antigen) level. The higher the PSA level in the blood, the more likely there is cancer; the test is usually used in prostate cancer screening tests. A Gleason score is the "grade" of the tumor, and is an important indicator of a man's prognosis after being diagnosed with the disease, according to the Prostate Cancer Research Institute. The lower the Gleason score, the lower the likelihood that the cancer has spread.
According to WebMD, prostate cancer tumors that are considered stage 1 are usually slow-growing, and may never actually present any symptoms. Because of this, some men may choose to go the "watchful waiting" route -- where they just keep tabs on the tumor to make sure nothing more happens -- or choose to have prostate-removal surgery or radiation therapy.
Stage 2 prostate cancer, on the other hand, can potentially spread outside of the prostate and cause symptoms if treatment is not taken, WebMD reported. Therefore, "watchful waiting" may be a good option for some men who don't seem to have any symptoms, but otherwise, prostate-removal surgery or radiation therapy (with or without hormone therapy), are the usual treatments. Cryosurgery (where tissue affected by the disease is frozen) is also an option, but WebMD pointed out that there's not enough information yet on whether the treatment works long-term.
There are two substages in stage 2 prostate cancer -- stage IIa and IIb. The substages are determined based on a man's Gleason score, PSA level and the extent the tumor has grown within the prostate, according to the National Cancer Institute.
Stages 3 and 4 are the more severe forms of prostate cancer -- in stage 3, the prostate cancer has spread beyond the prostate gland but only to nearby tissues, while in stage 4, the prostate cancer has spread to other organs, WebMD reported.
Risk factors for prostate cancer include being older (the disease is most common in men older than 65), being black, having a family history of the disease, and being obese, according to the Mayo Clinic.
Also on HuffPost:
More:Prostate Cancer Ryan O'neal Prostate Cancer Ryan Oneal Cancer Ryan Oneal Stage Two Prostate Cancer
HuffPost Lifestyle is a daily newsletter that will make you happier and healthier — one email at a time. Learn more