Preheat oven to 400°. Drizzle cut sides of spaghetti squash with oil and season with ranch seasoning, salt, and pepper. Place halves cut side down on a large baking sheet. Roast until tender, 30 to 35 minutes. Let cool slightly, then use a fork to scrape squash strands into a large bowl.
In a large skillet over medium heat, cook bacon until crispy, 8 minutes. Drain on a paper towel–lined plate.
Season chicken with oregano, salt, and pepper. Add to the skillet and cook until no longer pink, 8 minutes per side. Let rest on cutting board for 5 minutes, then cut into bite-size pieces.
To bowl with squash, add ranch dressing and ¾ cup mozzarella and toss. Add bacon and chicken and toss again. Divide mixture between the squash halves and place on a large baking sheet. Top with remaining ¾ cup mozzarella and Parmesan. Bake until warmed through and cheese is melty, 15 minutes.
Garnish with parsley to serve.
