No, you can't; you have to trade for an Oshawott from another player.
The counterpart to your gender will have had a tag battle with you in Nimbasa against the Subway Masters, so you will have seen Dewott, but you have to trade for the remaining starter to be able to register it.
You will also need to trade for a Rufflet and a Skitty, as they are White 2 exclusive Pokemon that do not appear with trainers in Black 2.