Place parsley, garlic, oregano, red wine vinegar, red pepper flakes, salt and pepper into food processor or blender. Process until finely chopped, stopping to scrape down the sides of the bowl with a rubber spatula as needed.
Add olive oil a little at a time while continuing to process. Stop to scrape down the sides of the bowl as needed. Transfer sauce to an airtight container and refrigerate at least 2 hours before serving.