O.J. SIMPSON is undergoing secret medi­cal tests for a cancerous brain tumor!

In a blockbuster world exclusive, The ENQUIRER has discovered the shocking details of the 66-year-old former football great’s devastating illness – and how, stuck in a Nevada prison, he’s made the most explosive confession EVER to the brutal slaughter of his ex-wife Nicole Brown.

Furious at a prison buddy who asked one too many times if he killed his wife, O.J. blurted out: “Yes, I killed the b***h – and I’d do it again if I had the chance!”

Now, as Simpson faces a lingering death in a prison hospital, The ENQUIRER has learned that the desperate fallen star has taken a final stab at winning his freedom – begging President Obama for a pardon so he can die in peace at home.

“O.J. says his memory is slipping and he gets painful headaches and other symptoms,” a close source told The ENQUIRER.

“He has blurry vision, stutters sometimes and has trouble hearing what people are saying to him. At times, he feels confused and has unusual bursts of anger and fits of depression. He put off the doctors for weeks after he first started experiencing the symptoms because he was afraid of what they would find.

“Now the prison docs are telling him they believe he has a brain tumor. They plan to run a series of tests on him, MRIs and CT scans.”

Simpson has finally agreed to the exams – and he’s convinced he doesn’t have long to live.

“I feel the end coming,” he told a pal. “But I don’t want to die in prison like common jailhouse scum.”

Simpson is deeply depressed because his legal pleas to be released from prison early were rejected by the court. Before he’s eligible for parole, he has to serve at least four more years of his original 33-year sentence for a bungled at­tempt in a Las Vegas hotel room to recover sports memorabilia he believed was stolen from him.

He was found guilty of kidnapping, assault, robbery and using a deadly weapon.

It’s unlikely Simpson will survive long enough to taste freedom again – his health was on a downward spiral even before physicians suspected he had a brain tumor.

As The ENQUIRER reported, only a few months ago prison doctors gave Simpson – a severe diabetic with very high blood pressure – just three months to live unless he dropped weight and exercised regularly.

“O.J. got really scared over what the doctors told him and began to follow a strict diet and exercise plan, which was a real struggle for him,” said the source. “But then he began suffering dizzy spells, nausea, lapses in memory and headaches – and the frightening symptoms continue to get worse.”

Concerned prison officials recently put Simpson on a suicide watch, but the source divulged: “O.J. told me, ‘Don’t worry, man. I’m not going to kill myself. I don’t have to – I’m dying. The docs think I’ve got a brain tumor. I won’t see another Christmas.’”

While the upcoming scans will determine if the disgraced NFL star’s death is imminent, his symptoms clearly match those the famed Mayo Clinic describes for brain cancer.

Victims suffer from frequent severe head­aches, nausea, double or blurry vision, problems with balance, speech difficulties, confusion, hearing difficulties and personality or behavior changes.

O.J. seems to have developed a hair-trigger temper. A fellow inmate felt its fury after he repeatedly questioned Simpson about the infamous 1994 slay­ing of his wife Nicole and her pal Ron Gold­man, which led to the “Trial of the Century” and the shocking not guilty verdict.

Outraged at the persistent inmate, O.J. exploded in a rage, confessed to killing both Ni­cole and Ron, and snarled: “She was messing with me!”

An insider says O.J. has hit rock bottom as he sits and rots at the remote Lovelock Correctional Center. To get there, visitors have to fly to Reno and drive 90 miles on treacherous mountain roads. His family members rarely visit him, dragging his spirits down even further.

“O.J. is miserable and he’s not thinking straight, probably a symptom of the brain tumor,” said the source. “He’s clinging to one last twisted hope – that President Obama will pardon him.

“He seriously believes the president will help him because he supported Obama and even contributed to his campaign.

“O.J. said, ‘I’ve been a supporter of Barack Obama since the beginning. He needs to give me a pardon!’” noted the source.

“He wrote to Obama, ‘Mr. President, I know my life is coming to an end. Please, don’t let me die in prison.’

“Of course, the president would never pardon one of America’s most infamous criminals, but it’s a desperate hope, and O.J. is clinging to it.”