He was actually created by Walt Disney himself, but not for the Walt Disney Company.
Here's the story:
* 1927: Walt Disney created Oswald for Universal Pictures.
* 1928: Disney lost the rights to Oswald to Universal after a contract dispute. This forced him to create Mickey Mouse.
* 1976: Disney reacquired the rights to Oswald.
So, while Oswald is associated with Disney, he wasn't created by the Disney company as we know it today. He was a product of Walt Disney's early career.