When installing the high-pressure switch in the rear head of the new compressor, is a new O ring required?

Usually, the O ring will be okay because it was new at the factory when they assembled the new compressor. For clarification I included a diagram so we can be on the same page. Check out the images (below). Let us know if you need anything else.

Thanks Ken.

You are welcome, please use 2CarPros anytime we are here to help.

I'll check the O ring for any sign of damage when I remove the plug.