I bought an expensive pure wild oregano oil oreganoworld very strong smell, 87.6% min. carvacrol, and it did nothing but SPREAD the fungus from 2 fingernails to all five nails on the same hand. I was also putting it in capsules and taking it internally. I eat no sugar & min. carbs, wear flipflops only, keep all nails dry as possible, also tried remedies: Vicks; ACV; baking soda, borax, iodine (internal & external, all 3); coconut oil; colloidal silver; garlic (burned heck out of toe); GSE ("max strength"); Listerine; Olive leaf extract (internal), I am allergic to rubbing alcohol, tea tree oil and neem oil. I have candida and nothing has worked on that -- food grade H2O2 when reaching treatment dose levels made my ankles swell (edema began so I had to quit H2O2 as edema kept getting worse), I tried niacin and my knees swelled up and became unstable. The candida has me under control, it seems. Cannot drink ACV or I get oral herpes outbreak.
What DOES kind of work for the nail funguses is hydrogen peroxide soaks and dipping or dripping it on multiple times a day. If I get the dose right it keeps the fungus sort of under control, at least the nails do not get maximally weird, although the fingernails are eaten away, & need to be clipped halfway down the nailbed, which is quite painful. I always bleach the clipper in between each nail.
Tried using oregano for a persistent toenail fungus in the large toe. Applied oregano oil diligently twice per day for 9 months and noticed no difference.