Heat 1½ tbsp oil in a skillet over medium heat. Add steak, season with salt and pepper, and sear until desired doneness, about 2-3 minutes per side. Transfer out of pan and set aside.
Heat 1½ tbsp oil in the same pan. Add onions, bell peppers, and oregano and cook until softened, about 4-5 minutes. Season to taste with salt and pepper.
Add steak and stir to combine.
Top with mozzarella, cover, and cook until melted, about 1 minute.
To serve, top lettuce with cheesesteak mixture.
