We've used internally but not for PN. It really must be diluted with a fatty oil because it is considered a "hot" oil and can irritate your skin. Make sure that you buy one already diluted. If you buy the pure essential oil of oregano, you will have to dilute it yourself and if you are not experienced using essential oils, then opt for one already diluted. Also, do a patch test on your self to see if your skin reacts. And, if you get it on a sensitive area, flush it with a fatty oil. Plain water will just spread it.
Zand is good brand to try. The other is called Oreganol. Both are diluted.
I'll have to try it on my shoulder and see if helps an old injury that's been causing trouble lately.