Residents around Mt. Moon were puzzled when several Clefairy were stealing all of their items. Oswald, however, proposed an interesting theory: the Clefairy were actually aliens and were using the belongings to help build their spaceship.
With the exception of Ash and his friends, nobody believed him. Along with Jigglypuff, the group searched for the Pokémon and discovered that they were indeed making a spaceship. Just as the ship was about to take off, Ash and the others fled. However, Oswald stayed with the Clefairy, wanting to learn more about them as aliens.
Although the Clefairy and their UFO reappeared in person in Wish Upon a Star Shape, Oswald has only reappeared in a flashback.
Professor Orville (Oswald) in The Electric Tale of Pikachu
In the manga series The Electric Tale of Pikachu, Oswald appears in the chapter Clefairy in Space. In VIZ Media's English translation, he has the name Professor Orville. His role in the manga is nearly the same as in the anime - he is a scientist who believes that the Pokémon Clefairy come from outer space.