Oppenheim developed a 'postquantum theory of classical gravity' in which spacetime remains fundamentally classical while quantum theory itself is modified, predicting stochastic fluctuations in spacetime that would manifest as an unpredictable, diffusive fluctuation in the measured weight of a precisely-monitored mass. He has proposed and is pursuing precision-mass experiments to test this prediction against the alternative (Bose-Marletto-Vedral-style) entanglement-witness route to probing the quantum nature of gravity, offering a theoretically distinct but experimentally complementary approach within UCL's quantum-gravity-sensing programme.