It depends if it is only on the surface of the nail, or it lives under the skin.
If it is only on the surface of the nail, apply oregano oil to the nail.
If it lives under the skin, take high doses of oregano oil internally. You may need to take high doses for months.
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Fungus only on the surface of a nail can actually be killed with hot water. The water needs to be as hot as possible without burning the skin. Have the hot water in a bath, and dip your foot in. Be sensible, don't burn yourself.