extracting in olive oil or other oils is just about the mildest extraction you can do. i suggest if you have time, allow the oils to absorb the herbal minerals for a long period of time and your material must be moisture free or you have to almost simmer it, until all vapors escape. if you allow even 1 drop of water in your oils, they will spoil.
the "only" purpose for this in general is in the making of ointments. you can experiement cheaply and you can only loose the cost of olive oil..