In a food processor or blender, combine the parsley, oregano, garlic, and red pepper flakes (if using). Pulse until finely chopped.
Scrape down the sides of the bowl with a spatula, then add the vinegar and salt. Pulse again to combine.
With the motor running, slowly pour in the olive oil through the feed tube until the sauce is smooth and emulsified.
Taste and adjust the seasoning, adding more salt or vinegar as needed.
Transfer the chimichurri sauce to a bowl or jar and let it sit at room temperature for at least 30 minutes to allow the flavors to meld.
Serve the chimichurri sauce alongside grilled meats, vegetables, or crusty bread. Enjoy!