The path is oscillator into a biquad low-pass into an ADSR into a gain stage, in that order, one voice at a time. The oscillator produces a band-limited waveform at the note frequency, offset by Octave and Detune. That signal runs through a second-order biquad low-pass, whose corner frequency is set by Cutoff and pushed around by Env Amt. The amplitude envelope then shapes level over the life of the note, and Master Gain scales what leaves.
The envelope is doing double duty: the same ADSR that controls loudness also modulates the filter cutoff by Env Amt. That's a deliberate simplification - real synths give the filter its own envelope, and if you want that, the clone-and-edit path below is about twenty lines of work. Sharing one envelope is why OSC Mini has eleven parameters instead of eighteen, and why the whole thing fits on one screen.
Two honest limitations. It is monophonic - new notes steal the voice, so legato playing retriggers rather than layers. And the filter is a single biquad, so it rolls off at 12 dB per octave and self-oscillates in the whistling-resonance way rather than the warm-analog way. Neither is a bug we plan to hide; both are on the list for OSC-1.
