Some antioxidants are not affected by heat, while others are. In the case of broccoli rabe, the difference in ORAC of the cooked versus raw is significant. The fresh rabe has about double the antioxidants compared to after cooking.
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