Since the clipping LEDs are back-to-back, it doesn't matter if they are installed backwards, as long as they are both backwards. In any case, that would not cause oscillation. Look for bad solder joints. When I do troubleshooting, I always start with a close-up visual inspection. It's usually the quickest way to find the problem. If one or more of the bypass caps are not connected, that would cause oscillation. Check C2, C4, C7 & C8. Looks like you kept the input & output wires away from each other, that's good. If you short the input to ground, does that stop the oscillation? Does turning any of the knobs change the oscillation pitch? Good luck!