OBDII is On-Board Diagnostics II, as in the second generation of OBD.  OBDI (the first generation) systems were often proprietary.  The automakers got together and semi standardized the system with the OBDII.

Now about your problem, if the vehicle in question is EFI (electronic fuel injection) and you have cut off the cat, then more than likely, the O2 sensor that is usually between the cat and the muffler is seeing that the cat is gone.  The ECU thinks there is still a cat that and is adjusting the mixture incorrectly.  This is all kind of surprising, since you said the 'idiot' light is not coming on.  Usually removing a cat on an EFI vehicle will cause the light to come on.

That's all I know to say.