Oscar the cat makes his grand entrances just as life is about to leave. A hop onto the bed, a fastidious lick of the paws, then a snuggle beside a nursing home patient with little time left. Oscar's purr, when keeping close company with the dying, is so intense it's almost a low rumble. "He's a cat with an uncanny instinct for death," said Dr. David M. Dosa, assistant professor at the Brown University School of Medicine and a geriatric specialist. "He attends deaths. He's pretty insistent on it." In the two years since Oscar was adopted into the third-floor dementia unit of the Steere House Nursing and Rehabilitation Center in Providence, he has maintained close vigil over the deaths of more than 25 patients, according to nursing staff, doctors who treat patients in the home, and an article in tomorrow's New England Journal of Medicine, written by Dosa. When death is near, Oscar nearly always appears at the last hour or so. Yet he shows no special interest in patients who are simply in poor shape, or even patients who may be dying but who still have a few days. Animal behavior experts have no explanation for Oscar's ability to sense imminent death. They theorize that he might detect some subtle change in metabolism -- felines are as acutely sensitive to smells as dogs -- but are stumped as to why he would show interest.