A one-time pad is malleable* and doesn't provide ciphertext integrity guarantees. If you want to add message authentication to a protocol, you still need something more than a one-time-pad.
* EDIT: Correct verbage
EDIT 2: Why are people continuing to downvote this after it was corrected?
* EDIT: Correct verbage
EDIT 2: Why are people continuing to downvote this after it was corrected?