Oregano is not a popular herb in Portugal except in the Alentejo, where it grows wild among the corks and olives. Portuguese country cooks favor sweet red peppers for this recipe, but for color I like to use a half-and-half mixture of the red and the green.
Preheat the oven to hot (400F.). Wash the peppers well, then lay them on their sides in a large shallow baking pan; bake uncovered 20 minutes, turn the peppers, and bake 15 to 20 minutes longer until the skins look papery and blistered; remove from the oven and cool until easy to handle.
Pull the stems from the peppers, quarter each pepper lengthwise, remove all seeds, then peel off the skins; pat the peppers dry on paper toweling. Arrange a single layer of peppers in a shallow broad-bottomed bowl, drizzle with half the oil and vinegar, then sprinkle with half the salt, oregano, and pepper. Layer the remaining peppers on top, drizzle with the rest of the oil and vinegar, and sprinkle with salt, oregano, and pepper as before.
Cover and marinate at room temperature 3 to 4 hours.