There are actually several poultry specific oregano supplements on the market and some are produced by well known brands like DurVet. I wouldn't risk essential oils, go with a well known poultry brand if you are going the oregano route. Also if your essential oils are eating through a cup they are most likely mixed with something else especially if you are buying cheap oils. They're not well regulated by any means. For example, if you see a brand that is selling rose essential oil for something like $20 run away from any and all of their products. It is HIGHLY diluted with something else other than rose oil. True pure rose oil will run you $200+ for 5ml-10ml due to the amount of rose petals required for extraction. I do not use essential oils so I am by no means promoting them however, I am absolutely fascinated with distilling things so I like to research these type of things.