For pure oil of oregano, I use the one sold by Natures Spices.
For the rest, I'll only use Young Living. They're the best I can find, although I think that for at least some of their oils, the quality isn't as high and some don't seem as potent as they used to be--for example, oil of oregano.
Still, Young Living is the best I can find and I won't buy from any other essential oil company.
My understanding is that often, essential oils are diluted and it's pretty well impossible to know what how they processed it and what is really in those little bottles!