Neither of them "founded a start-up" while living below the poverty line. Rowling wrote a book in her spare time that eventually got popular. She wasn't putting any money or asking of anyone else's money to fund her book.

Oprah followed the more usual path of "get a job, get promoted, rinse repeat" until she was a household name before she started her own company. Per the article you yourself linked, she was co-anchoring the local evening news at 19. That isn't the same thing as taking a risk on a start-up.