Place the parsley, cilantro (if using), garlic, lemon juice, oregano, red pepper flakes, salt, and pepper in the food processor and pulse to combine.
With the motor running, drizzle in the olive oil through the feed tube, scraping down the sides as necessary until everything is well combined. Taste and adjust the seasonings as needed.
Notes
Chimichurri sauce can be stored in the fridge for up to 2 weeks. Many people think it tastes better a few days after making it. Give it a good stir before serving each time.