Cook the pasta (rotini) according to package directions. Drain.
While the pasta is cooking, place the bacon in a large skillet set over medium heat. Cook until the bacon is browned and crisp. Remove the bacon from the skillet, drain on a piece of paper towel, crumble and set aside.
Discard all but 1 teaspoon of the bacon fat and add the olive oil. Add the onion and mushrooms and cook until the onion is tender and the mushrooms are just starting to brown. Add the garlic and dried oregano, and cook for 30 seconds.
Stir in the diced tomatoes, bring to a boil, then simmer for 5 minutes.
Add the cooked pasta, spinach leaves and cooked bacon to the tomato mixture. Toss to coat the pasta and wilt the spinach.