This paper reports experimental results demonstrating excellent short-term frequency stability of 45.6 µLHz (36 ppt@0.4 s integration time) for a bulk acoustic wave (BAW) silicon disk resonator oscillator. The n=4 radial mode of a BAW disk resonator demonstrates an extremely high-quality factor of 1.8*10^6 at 1.25 MHz. The disk is designed with anchors aligned with nodal locations to minimize anchor damping. The results on the measured short-term frequency stability reported here benchmark favourably relative to the state-of-the-art.