Chop the four sun-dried tomatoes into pieces and crumble the feta cheese. Break the three eggs into a bowl and beat them well. Season the egg mixture immediately by stirring through the oregano and a pinch of pepper.
Heat the tablespoon of olive oil in a skillet over medium-high heat. Add the baby spinach and sauté for about one minute until the leaves just begin to wilt. Pour the beaten egg mixture evenly over the spinach in the pan and let everything set gently.
Distribute the crumbled feta cheese and sun-dried tomato pieces over one half of the set eggs. Using a spatula, fold the empty half of the omelet over the filling. Let it rest in the pan for another thirty seconds so the cheese softens slightly.
Carefully slide the warm omelet from the skillet onto a plate. Serve immediately.