He gives you a certificate that says 'Player: (Your name) This document certifies you have met all the poke'mon in Sinnoh. GAME FREAK'
you will have to start the game again
The NPC that represents an actual developer from Game Freak the creators of Pokemon give the player a Diploma item. The Diploma is an in-game certificate congratulating the player for completing the Pokedex. In Pokemon Sapphire onwards you can receive two Diplomas, one for completing the Regional Pokedex and another for completing the National Pokedex.
what you do is go round and battle lots of different people and your pokedex will soon fill up that's what i did to fill up my and now i have completed the game.
you just have to play the game
Some Game freak designer guy will give you a ticket or a badge or something and you become one of the few to do it.
Congratulations you get a diploma from game freak at the celadon mansion and if you talk to oak well... he roars at you.
If you complete the pokedex you can get a diploma from game freak in the celadon mansion in celadon city and you can now say to everyone you did it!
You don't get the National dex in Generation I since there is only one generation of Pokemon. Once you fill you Pokedex, you can go to Celadon City, and go to the Celadon Mansion/Condominiums (Giant building next to the Pokecenter). Go up to the second or third floor, and talk to the Game Freak workers. You will receive a Diploma for completing your Pokedex. After that, if you have already beaten the Elite Four, you've pretty much conquered the game.
you get your legendery pokedex!!!! you get your legendery pokedex!!!! you get your legendery pokedex!!!!
Once you fill your pokedex for Proffesor Oak, you just wander around until you get bored of the game.
You have see to many different pokemons. Then you will fill it (you have to every Pokemon to get national pokedex).
see every Pokemon in the reigon that the game takes place in. for the national dex, trade and migrate.