Is the rear O2 sensor any different from the front mounted one?

Also, I'm looking at a OBX high-flow cat on ebay, but it doesn't have a hole for the sensor. Would I have to drill a hole myself or can I simply remove the O2 sensor?

 

The rear O2 sensor is the cat monitor, it may be the same as the front but I would find out for sure, I also believe they make kits to mount sensors in pipe that does not have a hole yet. Do both of your sensors have the same number of wires, for example 4 wire, then they are probably the same and if the rear is a heated sensor then it does not need to be mounted in the cat directly, you made your post sound like your rear sensor is in the cat now???

If you leave it out, you will get a check engine light, depending on how good this new type of can is, it may set a light if it does not work well anyways, even with two sensors properly installed. The car will still run good I think either way but I would like to have the second sensor and it would be needed if your state has emission testing. DO NOT toss the old cat.