I have been a foster parent in Canada for the last 5 years. A lot of the kids that come into care have lice. It has been an ongoing battle to get rid of the lice because they seem to have built up a resistance to the over the counter lice shampoos.
I now use about a half cup of olive oil with about 10 drops of oregano oil and coat in on the head. I usually leave it on over night. It kills the lice and eats at the glue that holds the nits to the hair follicle. It also breaks down the shell of the nit. I have found that the kids that I treat with this recipe don't seem to be so susceptible to getting lice for a long period of time