Whats the difference between an oracle and a decider in Computational Theory?
Oracles are entities capable of deciding even undecidable problems. Deciders, on the other hand, are essentially Turing machines (without an oracle) and therefore cannot solve undecidable problems (e.g. Halting problem).  




  I understand the point of a oracle is that we don't have to know how it gets its answer but it will always give us the right answer.


This is in fact the key difference.

An oracle doesn’t have an implementation, it is just a black box giving answer to any particular question (most importantly, the ones we cannot answer), whereas a decider has to be well defined Turing machine. In other words, we must know how a decider gets the answer, meaning it cannot answer any question.