It was reported in the New York Times back in 2012 that the commercial chicken farm, Bell & Evans, has been using oregano as a natural antibiotic for their flocks. They have been experimenting with various natural substances to keep their chickens healthy, instead of using antibiotics, and have found that oregano oil and cinnamon specifically seem to work better than anything they've tried so far.
The study and use of herbal medicine for humans and animals is a well-documented and time-tested practice that predates Western medicine by centuries. There's got to be something to it. But don't worry, you don't need a degree in holistic medicine or be an herbalist to incorporate herbs into your chicken keeping. Nearly every culinary herb has some great health benefits and is perfectly safe for your chickens.
Oregano is one of the most powerful natural antibiotics ever studied and has been found to be superior to many of the currently used antibiotics. The oil is more potent than the fresh or dried herb, but in any form, its hard to deny the power of oregano. No, not many scientific studies have been done on herbal remedies on chickens specifically. But that doesn't mean they don't work. Little by little it seems that more and more folks are seeking natural preventatives instead of turning to commercial wormers, antibiotics and other medications in an effort to raise their chickens as naturally as they can. We eat their eggs, so even more than other animals we raise, we seem to want our chickens to be chemical-, hormone- and antibiotic-free.
What we do know is that wild birds will line their nests with herbs, flowers and weeds. We also know that chickens seem to instinctively know what is good for them and what is not, what they need and in what amounts (think free-choice oyster shell or grit). So don't over-think things. Grow some herbs for your chickens. (Oregano, thyme, lavender and mint are especially beneficial and fragrant.) Then toss the fresh leaves into your coop and nesting box and let your chickens decide. Offer them fresh oregano as a treat. Dry some to crush and add to their feed through the winter. They'll eat some of the herbs, lay their eggs on some and ignore some, letting those dry and scent the coop. It's a win-win for you and for your chickens.