Can you get Oaks Letter in Pokemon Diamond without Action Replay?

You can't get Oak's Letter in Pokémon Diamond without using an Action Replay or similar cheat device.

Here's why:

* Oak's Letter is exclusive to Pokémon FireRed and LeafGreen. It's the starting item in those games, allowing you to travel to Kanto from Johto.

* Pokémon Diamond and Pearl are separate games in the Sinnoh region. They do not have any connection to Kanto or Oak's Letter.

While there are ways to trade Pokémon between games using the Pokémon Bank, it's not possible to transfer items like Oak's Letter.

Therefore, unless you use a cheat device like Action Replay, you cannot obtain Oak's Letter in Pokémon Diamond.