Here's the story:
* Walt Disney created Oswald in 1927 and he quickly became a popular cartoon character.
* Disney lost the rights to Oswald in 1928 due to a contract dispute with his distributor, Charles Mintz.
* Mintz hired a new animator, Ub Iwerks, who was previously working with Disney. Iwerks and Mintz then continued to produce Oswald cartoons.
* Disney went on to create Mickey Mouse in 1928 and he eventually eclipsed Oswald in popularity.
So, while it's true that Disney lost Oswald to Mintz, it wasn't a theft. It was a contractual issue.