Heat the olive oil in a nonstick skillet over medium heat until shimmering.
In a bowl, whisk the eggs with salt and black pepper until combined.
Pour the eggs into the skillet and cook undisturbed until the edges begin to set and the bottom is lightly set, about 1–2 minutes.
Sprinkle the shredded mozzarella, diced tomatoes, chopped spinach, and diced red bell pepper over one half of the omelet.
Fold the other half of the omelet over the filling and cook for another 1–2 minutes, or until the cheese is melted and the eggs are cooked through.
Carefully slide the omelet onto a plate and serve immediately.