Preheat oven to 400 degrees. Line a large rimmed baking sheet with parchment paper, set aside.
Cut each zucchini into halves through the length (if they don't lye flat trim a thin portion from bottoms so that they will lye mostly flat. I only had to do this with one of them). Pat insides dry with paper towels (cut portion). Align on prepared baking sheet.
In a bowl, stir together olive oil and garlic then brush lightly over tops of zucchini. Sprinkle with salt and pepper to taste then brush a slightly heaping 1 Tbsp marinara sauce over each zucchini, leaving a small rim near edges uncoated. Sprinkle tops evenly with mozzarella cheese then with parmesan cheese. Top with pepperoni slices (placing them more near centers as the cheese will melt and spread).
Bake in preheated oven 12 - 18 minutes (bake time will vary depending on how thick your zucchini are and how crisp/tender you want them).
Remove from oven and sprinkle with chopped fresh oregano. Serve warm.