How does this step up oscillator actually work?
It doesn't resonate - it's a step-up transformer (1:100). Each time the MOSFET switches on Vin gets transferred to the secondary with largely a 100:1 amplification in voltage just like a conventional transformer steps up or steps down (more usually) voltage.



ADDITIONAL INFO

Series resonation of the transformer's secondary leakage inductance and C2 cause the circuit to oscillate BUT, conceptually the MOSFET could be driven by an oscillator if one could be found to work at such low voltages.