The hole for the oxygen sensor is in the crossover (aka colostemy bag). This can be blanked with a M18x1.5 plug, stainless is best, put on quite tight, preferably with a crush washer to prevent vibration loosening it. Or use an aftermarket crossover - stainless tube, used to be sold by Agostini, Mistral & others for the V7's & Breva's - easier then to drain the gearbox oil. If you need dimensions, I could get that later today.
If it's your regular ride & you do significant miles on it, I would seriously contemplate having the headers ceramic coated. There are advantages, but it does cost.