Oak's Letter is a Key Item in the Sinnoh region games which allows the player to access a special section of Route 224 which leads him/her to the Flower Paradise. This item was introduced during Generation IV and with the letter it allows you to capture the Mythical Pokémon Shaymin. It is one of four Sinnoh based event items, alongside the Member Card, the Azure Flute and the Secret Key.
- Japan - April 18th 2009 to May 11th 2009
- North America - September 28th 2009 to November 8th 2009
- PAL - September 28th 2009 to November 8th 2009
- Despite the fact that Oak's Letter was programmed into Pokémon Diamond and Pearl by Nintendo, it was never officially distributed at any event.
- When Shaymin is successfully captured through this event, it is listed as having a Fateful Encounter even though it was caught in the wild. This is due to the fact that only a Fateful Encounter Shaymin can transform into Sky Forme in Pokémon Platinum version.
- When the Oak's Letter event is activated, Marley has different dialogue when the player speaks to her.
- Although this event was accessible through cheating in Pokémon Diamond and Pearl, since the first Generation IV games did not apply the Fateful Encounter tag to specific Pokémon, it would never be able to activate Sky Forme in the following game.