In 8-inch nonstick skillet, melt butter over medium-high heat. In bowl and with fork, stir together eggs, water, oregano, salt and pepper, just until mixed; pour into skillet. Cook for 3 minutes or until almost set, gently lifting edges with spatula to allow uncooked eggs to flow underneath.
Sprinkle half of the omelette with feta cheese and olives; fold uncovered half over top and cook for 2 minutes. Slide onto plate.