Oregano is anti-bacterial. Some commercial feeds now include oregano oil to help chick’s cope with coccidiosis. However, it cannot be relied on to prevent disease if the risk of infection is high.
Feed companies add the oil to feed in a form which prevents it oxidising, making it more digestible.
The best level of the main ingredient (carvacrol) is in oregano essential oil. In experiments with poultry, it was dissolved in vegetable oil and mixed into feed.