The oracle is just a circuit that negates one of the amplitudes. You specify it just like the rest of the circuit.
They use a trivial oracle circuit as an example. But Grover's algorithm works for any oracle circuit, so in principle it could be a circuit that verifies if its input is a proof of the Riemann hypothesis.
They use a trivial oracle circuit as an example. But Grover's algorithm works for any oracle circuit, so in principle it could be a circuit that verifies if its input is a proof of the Riemann hypothesis.