Oversampling is a method of preventing [aliasing] caused by lossy sampling or generation of waveforms. It involves generating the audio as if the sample rate was higher than it actually is by some multiple and then averaging the produced samples together to produce the actual output. The higher the oversampling multiplier, the better the quality but of course the higher the computational cost.
Oversampling is used in the [fm-synth]'s digital [[oscillator]] implementations to avoid [aliasing] at higher frequencies and [modulation-index]es.