How do you get oak letter in Pokemon Diamond and Pearl?

You can obtain the Oak's Letter in Pokemon Diamond and Pearl by catching a Munchlax. Munchlax is a Pokemon version of the popular snack "Mochi". Mochi is a pounded rice cake which is also a traditional gift sent with letters when travelling and is usually given by the Pokemon's mother or a relative when sent out to travel the region. This is why munchlax evolves so quickly with friendship because it is a representation of the trust it has with its trainer, similar to Ash and Pikachu in the anime.