i back SH
yes itll throw a O2 CEL
reason: the lack of restriction and backpressure upstream of the CAT causes a big drop in EGT, the O2 sensor needs to be at LEAST 600*F or more for it to give accurate readings to the ecu, the result is that the ECU detects the BS reading from the O2 and trips back-up mode (aka limp mode, aka full rich mode).

PS - a high flow cat is really a straight pipe, or at least mine is