Impact or thermal ignition of high explosives results in deformation that can lead to fracture. Fracture, combined with high pressure, dramatically increases the available surface area and potentially changes the mode of combustion. Recent impact and cook-off experiments on PBX 9501 (HMX, octahydro-1,3,5,7- tetranitro-1,3,5,7-tetrazocine, with a binder) have shown complex cracking patterns caused by impact or pressurization. Fast reactive waves have been observed to propagate through the cracks at about 500 m/s. We present experiments that investigate the propagation of fast reactive waves in cracks of PBX 9501, focusing on the reactive wave velocity and on the interplay of pressure and crack size. Experiments at initial pressures of 6.0 MPa reveal monotonic reactive wave propagation velocities of around 7 m/s for a 100 micrometer slot. We observe reactive wave velocities as high as 100 m/s in experiments at initial pressures of 17.2 MPa and various slot widths. Similar experiments at lower pressure exhibit oscillatory reactive wave propagation in the slot with periodic oscillations whose frequencies vary with combustion vessel pressure. This is the first reported observation of oscillatory combustion in cracks of an energetic material such as PBX 9501. Threshold pressure experiments for combustion propagation into closed-end slots of PBX 9501 find that combustion propagates into 2 mm, 1 mm, 100 micrometer, 50 micrometer, and 25 micrometer slots at approximately 0.1, 0.2, 0.9, 1.6, and 1.8 MPa, respectively.