Not exactly, metamorphic testing does not need an oracle. That’s actually the reason of its popularity in ML testing. It works by perturbing the input in a way that will produce a predictable variation of the output (or possibly no variation).

Take for example a credit scoring model: you can reasonably expect that if you increase the liquidity, the credit score should not decrease. In general it is relatively easy to come up with a set of assumptions on the effect of perturbation, which allows evaluating the robustness of a model without knowing the exact ground truth.