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You can really add anything to candles to scent them, as long as they don't contain alcohol or water.  The alcohol will evaporate when heated, and sometimes taking the scent with it.  And you know water and oil don't mix up.  You want it to be oil based.  Many times you can find fragrance oil in stores like Walmart, they are cheap, but you will be paying for what you get….

You can use food stuffs, too -- like vanilla beans, fresh herbs, rinds from oranges & lemons, cinnamon sticks, apples.  Whatever you use, you want to DRY it COMPLETELY first.  Water doesn't mix with wax either!  If you're using these whole, put them in a cloth bag and simmer in the wax for 30 minutes at 170F.  You don't want whole items to be in the candle -- they can catch fire! -- or you could put them around the edges.  If you're using crushed, you can leave them in the candle if you want, just pour them in the wax and let them simmer for a bit (maybe 10 minutes) to infuse the fragrance.  Those will color the candle, too!  Also there will be texture.

So, all in all, your peppermint OIL should be just fine!  I checked mine and all is says is Peppermint Oil.  I also checked my foodgrade oils -- Lorann -- online.  They contain little or no alcohol.  But for the price of these little 1 fl oz bottles, I'm sticking to buying my premium fragrance oils!  I can buy an 8 oz bottle for the same price of a 1 oz bottle of this candy oil!