The next paper is not in human study, but it does establish that oregano oil is effective at killing microorganisms. The authors experimented in the lab with several essential oils, to see how effective they were against a wide variety of bacteria, molds, and yeasts, and found that
“of the tested compounds, cinnamaldehyde, thymol, and carvacrol showed the strongest antimicrobial effectiveness”.
Vapor-phase activities of cinnamon, thyme, and oregano essential oils and key constituents against foodborne microorganisms.
López P, Sanchez C, Batlle R, Nerín C.
J Agric Food Chem., 2007 May 30, 55(11), 4348-4356.