The antimicrobial constituents of oregano oil are carvacrol (55 to 85 percent) and thymol (up to 10 percent). They kill bacteria by breaking their cell membrane [Source: PubMed].
Research tells us that oregano oil works to reduce different kinds of unhelpful bacteria, but leaves the friendlier Lactobacillus and Bifidobacterium alone [Source: PubMed]