you need to beat the elite four and then get the national pokedex for Oak.
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you have to get all eight badges from all the gym leaders.
He is in Pastoria in the house with the fat guy standing outside! REMEMBER you must have a heart scale in order to get a move!
you have to come back later, i think it is after you have finished the game.
you beat the pokemon league and if youre talking about the one on victory road you have to get the national pokedex
this is how u get throughbeat snowpoint city gym then get palkia if you have Pokemon pearl or go and get dialga if you have Pokemon dimond then try again then you should be able to get there. ^.^ hope this helps people who read this and it did not work!