When this item is received as an event and delivered by the deliveryman at the Poké Mart, the player will find Professor Oak waiting for them on their next visit to Route 224, at the northernmost, barren end of the route. He will explain to the player that the white rock there is present for a person to note what they are most thankful for, and upon inspection of it, the player will be able to enter their own note (via the same text input system as is used for Pokémon nicknames). After this has been done, the Seabreak Path will appear, and a Shaymin will come down from it before returning north to the Flower Paradise, where it can be caught at level 30.
Despite Oak's Letter only being distributed for Pokémon Platinum, the Wonder Card for the Pokémon 20th Anniversary Shaymin states that, "With a special letter, Shaymin could first be encountered in the Flower Paradise of Pokémon Diamond and Pokémon Pearl."
Marley will speak to the player differently with the Oak's Letter event activated.