I don't think there are any Old Ones in the show, there are only the Originals and Silas, both of whom were humans before becoming vampires (directly via magic, as opposed via blood or perhaps a bite from other fictions).
I haven't read the novels but the idea that the origin of how the Old Ones truly come from is unknown makes it interesting, I like mysteries.
Two theories regarding the Old Ones: Either they were born as vampires via evolution, possibly indirectly affected by magic or maybe they were created via magic, they were never born the conventional way.