You can't.
You have to defeat the elite four also. Then go back to Prof. Oak, and he will give you the national pokedex. Hope that helped....
Prof oak will always be in his lab in pallet town.
you have to see all of the Pokemon and then you get the national pokedex from Prof. Rowen and Prof. Oak in Prof. rowen's lab after completeing the Sinnoh pokedex.
go and meet prof. Oak
What to do after you talk to prof oak about groudon
He should have 2 aids standing right inside the door at his lab in Pallet Town.
you have to do prof. oak in bed then he gives you the Oak mail you have to battle prof. oak then he gives you the Oak mail
prof oak
Prof Birch is NOT in Diamond Prof OAK is in Eterna City.
You cannot get the Black Apricorn from Professor Oak.
You get it from Prof. Oak once you beat the elite four. (You do not need to do anything, Prof Oak takes you to his lab and gives it to you)
You don't battle Prof. Oak.
in eterna city
talk to prof oak
use cheats
you dont