My check engine lite came on after some mild hesitation during part throttle and gear change intervals.

My suspicion is the O2 sensor is going out of whack (still original one from factory). Could there be anything else to look out for? Haven't gotten the fault codes out yet, the dealer wants $$ for it and I'm gonna take my guesses first before pulling codes.

It could be anything, it's worth getting the codes read.

I had a similar experience and the code was for "rear knock sensor". The fault would happened during acceleration and I traced it down to distributor cap. After changing the distributor and the rotor the fault went away.
Hope it helps.

You have 2 O2s and it's highly unlikely that one or more is causing hesitation. It'll be cheaper to pay diagnostic labor than it will to guess at costly O2 sensors which probably won't solve your problem but check for obvious vacuum hose leaks first, before doing anything. Unless it's a code for something like a loose gas cap, it won't reset by itself so it'll still need to be read and reset by a scan tool anyway if you want the light out.

Mine is an Asian S70 with one O2 sensor before the cat only. I've swapped my ECU back to the stock one (yeah I got two ECUs). Will see whether the light shows with the stock ECU, I'll get the O2 sensor heat cleaned first before pulling codes.