Heat olive oil in a frying pan over medium heat, add the cherry tomatoes, and cook for 4-5 minutes until softened and blistered.
Whisk together eggs, milk, oregano, salt, and pepper in a bowl, then pour the mixture over the tomatoes in the pan. Gently shake to distribute evenly.
Scatter spinach and crumble feta on top. Cook for 3-4 minutes until the edges set, then cover for 2-3 minutes or fold the omelette to finish cooking. Serve hot.