Combine parsley, cilantro, and garlic in a food processor and pulse to chop.
Add oil, red wine vinegar, 2 tablespoons water, oregano, kosher salt, and red pepper flakes. Pulse to combine.
Cover with plastic wrap and let stand at room temperature for at least 1 hour.
Notes
To substitute dried oregano, use 2 teaspoons dried oregano and mix with ¼ cup hot water and allow it to hydrate for 10 minutes.
This sauce can be prepared and refrigerated for up to 3 days. It is important not to use the sauce after the 3 day mark because it contains fresh garlic.
Garlic, which has extremely low acidity, could produce botulism (a serious toxin) when stored for too long, and that is not a risk worth taking.
Bring sauce to room temperature before serving.
Depending on taste preference, the cilantro may be omitted. Simply use more parsley in its place.