This is one of the times I can't help but shout "get a dictionary, you two", but in short there are more than those two terms and in this case they're mutually exclusive. A "Seer" supposately sees the future, just like when you sit in the toilet for long you end up hearing a popping sound. An "Oracle" and a "Prophet/Prophetess" are tied to a deity but their functions are different: in Greek mythology, an Oracle knows the will of the gods and uses that wisdom to advise others, whereas a Prophetess gets revelations from the gods about the fate of their people.