Mince 4 garlic cloves and place them in a medium bowl, 2–3 minutes.
Add 2 tsp dried oregano, 0.5 tsp red pepper flakes, 1 tsp kosher salt, and 0.25 tsp black pepper. Stir in 3 tbsp red wine vinegar and 3 tbsp water, then let stand 5 minutes to hydrate the oregano.
Finely chop 1 cup packed flat-leaf parsley and add it to the bowl; stir until evenly combined, 30–60 seconds.
Stream in 0.5 cup sunflower oil while stirring until the sauce looks glossy and spoonable, about 1 minute. Do not puree; chimichurri should be loose with visible bits of herbs and garlic.
Let the sauce rest at room temperature 20–30 minutes for flavors to meld. Serve, or refrigerate covered up to 5 days; bring to room temperature and stir before using.