To turn on an oscillator simply turn up or down the "Volume," knob found in OSC 1, 2 or 3:
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When an oscillator is on the pink light at the top right of the oscillator section will also be on. In the example below oscillator one and two are on, but three is off:

To turn off an Oscillator simply drag the "Volume," knob all the way to the left. This will turn off that single oscillator.