The 6 foot 10 inch O'Neal was a four-star recruit out of Crossroads High School in Santa Monica.
"Recently I found out I had a serious heart issue," O'Neal wrote on Instagram. "I am very blessed to be here living today."
He was part of a UCLA recruiting class ranked third in the country behind Duke and Kentucky.
Another UCLA player will also be sidelined this season: Forward Alex Olesinski was diagnosed with a stress fracture in his right foot that will keep him out for two to three months.
O'Neal says he expects to return to the court after a medical redshirt season in 2018-2019.