Bring a large pot of salted water to a boil and cook the penne pasta until just tender. Drain and set aside.
Heat the olive oil in a skillet over medium heat. Add the diced onion and cook for three minutes until it softens.
Stir in the minced garlic and cook for one minute more. Add the zucchini slices, salt, pepper, and oregano. Cook for five minutes until the zucchini begins to soften.
Pour in the crushed tomatoes and simmer the mixture for three minutes. Remove from heat.
Combine the cooked pasta with the tomato and zucchini mixture in a large bowl until evenly coated.
Transfer everything to a baking dish and spread it into an even layer. Sprinkle the shredded mozzarella cheese over the top.
Bake at 375 degrees for 15 minutes until the cheese is melted and lightly golden.