Hirata

Oswald is a character of the day who appeared in Clefairy Tales, as well as in The Electric Tale of Pikachu.

Oswald is a space enthusiast residing in a small town in Kanto, though his technology, which he purchased via comic books, failed to work. When the townspeople were puzzled by several Clefairy stealing all of their items, Oswald proposed the Clefairy were actually aliens who were using the belongings to help build their spaceship. However, nobody believed his theory except for Ash and his friends.

Along with Jigglypuff, Oswald and the others searched for the Fairy Pokémon and discovered they were indeed making a spaceship with the stolen items. Just as the ship was about to take off, Ash and his friends fled. However, Oswald stayed with the Clefairy, wanting to learn more about them as aliens.

Though the Clefairy and their UFO reappeared in Wish Upon a Star Shape, Oswald was not physically seen and only reappeared in a flashback.

(Source: Bulbapedia)