because he is on holidays :D
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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.
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.
First, get the National Pokedex. Then,talk to professor oak in eterna city.(he is at the bottom left corner of town as far as i know lol)
in the house beside enterna citys gym
Professor Oak will not appear in the Lab in Sandgem Town until you complete the Sinnoh Pokedex. He will reward you the National Dex when you arrive there.