sensors

Was looking at a decat and shotgun pipes for the 328 drift car. Question is can I run without O2 sensors in place or will it be down on power and run shit or is it just an emission thing for the nct that they need to be in for. All info welcome

Found out today one of my reversing sensors is on the blink and got a quote of €166 from the stealers for a painted one! Just wondering if anyone has any experience from some of the suppliers of new but NON oem sensors on ebay in the links below...