An oscillator is the circuit or function in a synthesizer that generates the periodic waveform a sound starts from. It is often abbreviated as OSC. You pick a waveform such as a sawtooth, square, triangle, sine, or noise, and then shape the tone from there with filters and envelopes.
The waveform is the starting point of the tone
A sawtooth wave contains many harmonics, so it is an easy starting point for bright, powerful sounds. A square wave has a thick, hollow character, while triangle and sine waves make gentler textures. A waveform on its own is not a finished sound; you use it together with the filter and the level changes that follow.
Setting the pitch
The frequency of the oscillator determines how high the note sounds. When you play a key or a MIDI note, the oscillator frequency normally changes to match that pitch. Detuning several oscillators slightly adds thickness and movement, but check that the result does not become muddy.
Layering several oscillators
By stacking two or more oscillators and dropping one an octave, changing its waveform, or changing its level, you can build complex tones. The more you layer, the louder the sound and the more crowded the low and mid ranges become. Start with one oscillator and add only what you need.
How it relates to the filter
Cutting the rich harmonics from the oscillator with a filter such as a low-pass changes the brightness and the sense of front-to-back distance. Moving the cutoff or resonance can change the impression greatly even with the same waveform. Checking the filter settings before you swap waveforms makes the cause easier to find.
How noise differs
Noise is a source without a definite pitch, used to add wind, breath, or the attack component of a hit. On many synths it is provided separately from the oscillators. For the shared basics of sound design, see the article on software instruments.
