I use Now Brands 100% pure essential oil. Bought it from Amazon. For a diffuser I have a metal tea ball that I put in front of a small fan to spread the vapors through the air. I put 4-5 drops of oregano oil on a small piece of paper towel that I put in the tea ball. When I came down with whatever I had, I added the 4-5 drops a half dozen times through the day. Every couple of hours I replenished the oil on the paper towel. Added new oil just a couple of times the next day to knock down whatever germs might still be hanging around, even though I was feeling better already. I haven't used it since. I haven't been using it as a preventative.
I use it in a diffuser because I wanted to get the antiviral and antibacterial effects into my respiratory system where I assumed the germs were taking hold. I thought about taking some orally as well, but decided not to. I figured that it has such good antimicrobial properties that it would probably kill off too much good gut bacteria. Then I'd be no better off than people that get a prescription for antibiotics from their doctor.