In a 10-inch skillet, heat oil over medium-high heat. Add onion and spinach and cook, stirring occasionally, until onion is translucent—about 5 minutes. Season with salt and pepper.
Add eggs and chopped bacon, season with salt and pepper, then stir mixture. Let cook undisturbed until the edges of mixture begin to set, about 2 minutes.
Transfer to oven and bake until top of frittata is set—about 10 minutes or so.
Cut into wedges and serve. Can be served hot or at room temperature.