Robust authentication and integrity for short OTP message
If you want to be consistent with OTP usage, then you should use a One Time MAC. Currently it seems the easiest path for that is to simply use Poly1305, but remember that you must never reuse keys for Poly1305, just like you can't reuse keys in OTP. (In particular, don't use the key generation routine used by ChaCha20-Poly1305 to derive the Poly1305 key from a nonce/key pair).

Using HMAC doesn't make much sense: if you're using OTP, that's probably because you don't trust any particular block cipher, so you shouldn't trust any particular hash or MAC algorithm.