In an FM algorithm, all oscillators are capable of producing audio output.
In a regular FM algorithm diagram, the oscillators that are audible appear at the bottom.
In this simple 4-operator chart, you can see that in the first algorithm (1), only oscillator 1 is audible while the rest are modulating its output. However, in the last algorithm (8), all oscillators are audible together.
All of the oscillators still operate within the audible range, whether directly outputting sound or modulating another oscillator. This is what enables FM synths to produce those signature combinations of complex shifting waves and harmonics. The bottom oscillator in each algorithm can be thought of as the fundamental frequency, key in producing the pitch quality in the sound. Any other oscillators above the fundamental can be assigned to either track the pitch similar to a harmonic or stay fixed like a ring modulator.
On top of that, each oscillator can be assigned independent envelopes along with a few other parameters, like a very limited mini synthesiser all by itself. These mini synths are what we are referring to when we talk about "Operators" - They're not just oscillators but more complex units.
A single operator in Dexed VST looks like this:
You can see that this is not just a simple oscillator, yet this is probably one of the most simple implementations of an FM operator.
An FM synth may also have one or more LFOs that operate below audible range and can be used to modulate pitch, amplitude or filter width in a more familiar way to give vibrato, tremolo, etc.
This is very similar to the way "West Coast" synthesis works.