You can't catch or obtain an Oshawott.I had the same Problem with the Tepig Line.I chose Oshawott for my Starter and my Rival Chose Snivy.So I wasn't able to see the Tepig Line in the game.So,I traded my Emboar from My Black to my White 2, Bred it with a Ditto and Tada ! Tepig is Borned.You should do the same thing.