I have the first MOT on my XVS 1300 next month, I fitted an O2 mod a few months ago. Does anyone know if I need to reconnect the original sensor for the MOT.

I've not had mine MOT'd yet - but I don't think it would. I imagine the 1300 even with an O2 sensor would give less emissions than my old 2001 Fazer. I asked about the Cobra Slip-on (no cat) and they said so long as it doesn't say 'not road legal' or isn't deemed too loud it should pass.