An herbalist will not usually recommend taking oregano oil or any essential oil internally (orally) because it is extremely anti-microbial. It is so anti-microbial that it will kill a significant amount of your good bacteria in your guts that you need to digest, maintain your immune system, reduce intestinal permeability, etc. So if you are taking essential oils internally, make sure to take probiotics with it too, just like you would or a pharmaceutical antibiotic. The thing about essential oils is that if taken for long enough or at high enough doses they can become very irritating to the digestive tract (they are literally solvents that destroy lipid-based cell-membranes, and they don't differentiate between your cells and pathogenic cells) and when they are flushed out through the kidneys, they can damage kidney cells if in high enough doses. However, if you take them cautiously, for a short period of time at a reasonable dose, or intermittently to allow your body to flush it out for a period of time, it may be safe. All this said, taking essential oils internally may be a lot safer than taking a course of IV antibiotics, but the difference is that when you're in the hospital they are regularly checking your kidney and liver functions, something we can't do at home when taking herbs on our own. That's another reason why getting in touch with a good herbalist is important, so you can get the doses right.
Another thing: low-molecular-weight volatile oils in herbs, like the thymol molecule in thyme, are specifically good for the lungs because they can pass from the guts into the blood and then into the lungs where they are breathed out, having an antimicrobial effect on the lungs directly. However, thymol can also have toxicity at higher doses, so it's not recommended to take it orally. Steaming with it may be safer and have a direct impact on the lungs (like an old-fashioned nebulizer). But steaming with other low-molecular-weight essential oils like eucalyptus and tea tree oil is considered safer than with thyme oil. I just bought some eucalyptus oil yesterday and will test it out on myself by steaming it. Also, taking herbs in their whole form (and not just the extracted volatile oil) is much safer and does not have the same problems with toxicity. Taking a thyme or eucalyptus tincture, for example, might be just as helpful, and you might be able to take that for a longer period of time.
Oh, and there is essentially no risk of antibiotic resistance with essential oils (or any whole herbs for that matter) because they are too complex for bacteria/pathogens to figure out a way around them. I saw a comment somewhere in this thread about that, so I just wanted to allay that fear. I believe the reason we would rotate herbs is not to reduce the risk of antibiotic resistance, but instead to let the body cleanse the compounds that could build up in the body and become potentially toxic at higher doses.
That's my two-cents. Gawd do I love herbalism school!