The ECU is probably running properly now that you have cleared the faults, which is why the O2 sensor is showing up all of a sudden. You know there were other wiring problems, so I would check the O2 sensor wiring too. If you find out that you need a new sensor, get a OEM Honda sensor instead of an aftermarket sensor from Autozone. I know the Honda sensors are more expensive, but they actually work properly.

The guy you bought it from must have been pretty intelligent if he messed up an LS swap in an EG. It doesn't get any easier than that when it comes to engine swaps.