I use Young Living essential oils both internally and externally. For the stronger oils, I'll put a drop or 2 in 1/2 cup to 1 cup of distilled water and drink. It says on the label if it's safe for internal use and also on their website. If you need more info, let me know and I'll look it up in my Essential Oils Desk Reference for you.
I use a carrier oil for external use; the essential oil goes farther and seems to absorb better. Right now I'm using the Young Living V6 oil or Aura Glow (Heritage Store) product.
One of the best essential oils is oregano. Young Living's isn't as good as it used to be, IMHO. So I buy my oregano from MH now. It can be used internally.