It’s a real thing, but it’s not an apple and doesn’t grow on oak trees. It’s a nickname given to a gall created by a wasp.
Fake folklore has it that oak apples are natural aphrodisiacs and farmers used to collect them for their livestock before switching over to artificial insemination. Some farmers may have tried a few themselves, too. Farm families are unusually large (12-20 children) where oak apples are common and the farmers are among the most cheerful people in their region.
People are high discouraged from experimenting with them. Oak apples may be poisonous, they may contain parasites and may even be hallucinogenic.
Don’t eat four of them, parboiled with 1/4 teaspoon of nutmeg and a crushed clove. You’ll see the inside of the universe and may forget your name for a few hours.
Nothing has ever been proven about oak apples. There is no data. Research is banned in most states.
Scientists continue to insist there is no such thing as a natural aphrodisiac and that people should continue to buy actual, proven boner drugs on the internet or from talk radio hosts.