Look closely, they don't have the same nose nor eye shape. Their prominent teeth aren't the same. Oshawott has flipper-like feet, Chespin has claws on his feet. Chespin has discernible digits on the front paws. Chespin has a longer, pointed tail, Oshawott has a short, broad tail. Oshawott based on a mustelid, chespin based on a cross between hedgehog and rodent.
The only similarity I see is the round head and the open-mouth smile (which also has a different shape)
Bazinga! *dives into ballpit* Pokemon Black Friend Code: 0605-2800-9952; Trainer Name: Solace; First Pokemon (roleplaying): Shiny Umbreon