Providing you have a pure undiluted oregano oil, the standard method is to mix it with 12 parts olive oil, this is Dr. Cass's "Super strength Wild Oregano Oil" that sells for a whopping $74.95 an ounce!!!!!!!!!!!!!
The pure stuff you may have, I put 1 drop in 1 gallon of water and when you use to that, you can add 2 drops.
In clinical use I know one man that puts a very potent mixture directly into the cheek of 3-4 week old babies that are dieing of whooping cough and he claims a cure in 7 hours and has done this many times. This sounds too powerful to me and should be left up to a professional. No matter how you use it, it is the strongest fungal killer you would ever want or need. If oregano oil fails? I just can't imagine it could!
If you put 1 drop to a gallon of liquid drink, children soon drink it and never say a thing about the taste, it only takes 1-3 days and you will not know it is in there.
Always remember READ before you experiment! Never put a concentrated herb especially an essential oil directly in the mouth unless your prepared for what can happen!