The tabloid said its source also told it Simpson, angry at another inmate at Lovelock Prison who kept asking whether he killed his ex-wife, Nicole Brown Simpson, said he did.
"Yes, I killed the [expletive] and I'd do it again if I had the chance!" he reportedly said.
Simpson, 66, who was acquitted in 1995 of killing his ex-wife and her friend Ron Goldman, was sentenced to 33 years in prison for robbing a sports memorabilia dealer in Las Vegas in what he said was an attempt to recover property stolen from him. He was recently told he must serve at least four more years before parole.
The source said Simpson has symptoms consistent with a brain tumor.
"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," the source said. "He put off the doctors for weeks after he first started experiencing the symptoms because he was afraid of what they would find."
Simpson reportedly told a friend he does not want to die behind bars "like common jailhouse scum." The source said he has "one last twisted hope," a presidential pardon.
"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," the source said.