Quantum oracle in Grover's problem?
When computing the oracle in Grover's algorithm, you prepare a qubit entangled with the search register such that the qubit is ON in the parts of the superposition where the search register contains a satisfying solution and OFF in the other parts of the superposition. This is not hard to do: you just use any old circuit that would determine if a given input was a solution, and apply that to the search register.

By preparing the solution qubit, applying a Z gate to it, then uncomputing the solution qubit, you negate the parts of the superposition corresponding to solutions. Having that qubit available is not the same thing as finding a solution. For example, if you measured it you'd almost always get an OFF result and find that your search register contained a non-solution. But, by using the prepared qubit as a tool to negate the phase of the solution space, Grover's algorithm can set up a gradual rotation towards the solution space.

More information: http://algassert.com/post/1719