The Owl and the Pussycat were friendly in the classic poem, but real life doesn't always reflect art. Owls are not social animals even with their own kind -- except for the couples who are nesting and raising a family. Considering cats are predators, it seems to go against nature, but depending on the circumstances, it's more likely that an owl would go after a house cat than make friends with her if he were hungry.
It Can Happen
Owls are efficient, opportunistic hunters. If times are tough enough, almost anything that comes into view of a hungry owl is considered fair game. That means owls are a danger to your house cat if there's a shortage of the prey that is their typical, natural source of food. This can become a problem, especially during the nesting season when owls are raising young. If they have extra mouths to feed and owls' usual diet happens to be scarce, they will go after house cats and other animals that aren't on their normal diet.
Owl's Natural Prey
Your house cat should be safe from the neighborhood owl if the rodents and other mammals he prefers to hunt and eat are in plentiful supply. An owl's natural prey list is rather extensive; he'll kill and eat just about anything he can haul away. Owls' prey animals include mice, chipmunks, moles, shrews, gophers, rats, squirrels, weasels, raccoons, birds, frogs, snakes, fish, skunks, rabbits and even porcupines.
Cats Will Turn the Tables
Just because cats can sometimes become prey for owls doesn't mean that your kitty won't go after an owl. Birds are, after all, natural prey for cats. In an article for the University of Texas at Austin, Chris Johnson reported that over a period of two breeding seasons, house cats killed 10 owls he was studying. Both adult female owls of the breeding pairs were killed, so it's not just baby owls who are in danger from cats.
Keeping Kitty Safe
You shouldn't consider your cat's feistiness and prowess as a hunter insurance against an owl going after her. In his book "Great Horned Owl," Dwight G. Smith recounted the story of how Chuck LaRue watched a great horned owl attack and kill a bobcat. That should give sufficient cause for you to be extra careful with your kitty if you know an owl has taken up residence in your neighborhood. Whether you know for sure or not, it's still wise to bring Bootsie indoors at night. She should be safe outside during the day, but since owls start hunting in earnest after sunset, cats aren't safe from them during the nighttime.