Preheat oven to 450 degrees F. In a 13x9x2-inch pan combine potatoes and red onion. Toss with olive oil and sprinkle with salt. Roast, uncovered, for 15 minutes.
Meanwhile, in a medium bowl whisk together eggs, cheese, and a pinch of salt until combined. Reduce oven temperature to 400 degrees F. Pour egg mixture over vegetables; return to oven. Bake for 7 to 9 minutes or until eggs are set.
Invert frittata onto a large cutting board; cut in wedges. Flip wedges top side up and place on plates. If desired, top with oregano.