You have basically completed the game. You can also get the 151st Pokemon (Mew) but you can only do it through an in game glitch (search it on YouTube) or by using a cheat device. You can also go to Celadon City and go to the Large Building and talk to the Game Designers the game creater will give you a diploma whether you have caught 150 or 151 Pokemon.
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Professor Oak gives you a diploma...that's pretty much it.
Keep catching, there's more! Go to Professor Oak to get your Pokedex evaluated.
For a player to complete the Pokedex in the first generation games Red and Blue, they must catch all 150 official Pokemon from the first generation. After this, they receive an in-game certificate and congratulations from Professor Oak.
Well when you SEE all 150 Pokemon prof. Oak will give you a deploma then go to cinnibar island and head inside and talk to bill...and be ready for an adventure!! Then when you own all 150 Pokemon you can get the mystic ticket! (which you can only get in an event) this allows you to get ho-oh and lugia! yipee
Here is a list of all 150 Pokemon: http://www.serebii.net/diamondpearl/shinoudex.shtml The last Pokemon listed (#151) is not required to get the national pokedex.