I believe you meet him after completing the Sinnoh Pokedex. Then he will update your pokedex to national.
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At the beginning of the game, Professor Oak is in Mr. Pokemon's house. Mr. Pokemon's house is north of Cherrygrove City. After that, Professor Oak is in Pallet Town in his Pokemon lab.
What the hell is a Pokemon crystal
In Pokemon Platinum to get the National Dex you will need to have seen all 210 Pokemon in the Sinnoh Dex then talk to Professor Rowan and then after a while Professor Oak will give you the National Dex!
You can trade from Platinum to SoulSilver as soon as you have received the regional Dex from Professor Rowan in Pokémon Platinum and as soon as you have received the regional Dex from Professor Oak in Mr. Pokémon's House in Pokémon SoulSilver.
in Route 224