Combine hot water, oregano, and salt in small bowl; let stand for 5 minutes to soften oregano. Pulse parsley, cilantro, garlic, and pepper flakes in food processor until coarsely chopped, about 10 pulses. Add water mixture and vinegar and pulse briefly to combine. Transfer mixture to medium bowl and slowly whisk in oil until incorporated and mixture is emulsified. Cover with plastic wrap and let stand at room temperature for at least 1 hour. (Sauce can be refrigerated for up to 2 days; bring to room temperature and rewhisk before serving.)