As is the case with many Italian herbs and spices, oregano contains a high amount of antioxidant content. Though if you want higher, try switching out with similar tasting marjoram, which ranks even higher on the ORAC scale.
ORAC Source
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010