You can't unless you picked it as starter pokemon.
Oshawatt is a Pokemon only available in Pokemon Black and White. There is no way to get it in Pokemon Platinum, because that Pokemon didn't exist when the game was made.
Starters are Oshawatt Tepig and Snivy
There is no way of getting all beginner Pokemon without trading.
You dont
You cant without action replay
You can't without trading with someone that has black.
Not without trading from another game.
You can't, you have to choose it as your starter. this is the same for Tepig and Oshawatt.
no it is immpossable unless you have an action replay with the Pokemon modifier code
Diglett can not be obtained in Black and White without trading or cheating D=.
Snivy, the grass starter. Tepig, the fire starter. Oshawatt, the water starter.
As of March 5th,2011 ash has caught a Pidove and a Tepig and Oshawatt has joined his team.