[Lomelinde]

In order to answer why Bombadil didn't fall under the spell of the ring and desire to use it, one must first ask the question who was Tom Bombadil? Tolkien kind of left that up to the reader, and many have suggested that he was a Valar(manwe, or something). I personally have absolutely no idea but there are lots of people who have their own theories about the elusive Bombadil. Perhaps the kind of being he was had to do with the innate resistance to the evil of the ring he possessed.