XOR and Caesar are different ciphers, which encipher plaintext using a key. One-time pads are used in conjunction with a cipher to enhance security. A one-time pad is a series of randomly generated keys for a cipher. Each character in plaintext is enciphered with the next key given by the pad. This becomes secure because the characters are enciphered with different keys, rather than the same one, and makes cryptanalysis much more difficult (if close to impossible). A copy of the original pad is needed in order to decrypt the ciphertext. Where you have to be careful, is making sure the one-time pad is secured where no-one else can get it, but that the proper recipient has to have a copy in order to decipher/encipher messages.
Theoretically, which cipher is used (XOR, Caesar, or other), shouldn't matter with a one-time key, but XOR is more likely to be used (computationally faster than Caesar).
