I would like to simulate an oscillation movement of 120/100 degrees around a fixed point.
Something like this one:
Basically, I want to rotate a gear around its axis, but only have a partial rotation and then have it return back.
What you're looking for is reciprocal motion, or to convert rotation motion into linear motion (and then back to a limited rotation).
Building out the key parts of the fan head you have the following setup (using fairly standard technic pieces:
Adjust the distance between the gears and the size of the gears as you require. If you need a larger gear in (3) you could replace the gear with one of the liftarms with an axle hole rather than circles, etc.