Preheat the broiler. Whisk the eggs, 2 tablespoons water, a big pinch of salt and a few grinds of pepper in a medium bowl. Heat 1 tablespoon olive oil in a large nonstick ovenproof skillet over medium-high heat. Add the garlic, scallion whites and oregano and cook until the scallions are golden, 1 to 3 minutes. Add the egg mixture and stir with a rubber spatula to evenly distribute the scallion mixture. Draw the edges of the egg toward the center of the pan with the spatula a few times. Reduce the heat to medium low and cook, undisturbed, until the edges and bottom are set but the top is still wet, about 4 minutes.
Sprinkle the tomatoes, 1/2 cup feta and the olives on top of the frittata. Transfer to the broiler and cook until the top is just set and the edges are golden, about 4 minutes. Slide the frittata out of the pan onto a cutting board and let rest 5 minutes.
Meanwhile, whisk the remaining 2 tablespoons olive oil, the vinegar and a pinch each of salt and pepper in a large bowl. Add the lettuce, dill and scallion greens; season with salt and pepper and toss. Sprinkle with the remaining 1/4 cup feta. Cut the frittata into wedges and serve with the salad.