Carefully slice the spaghetti squash in half lengthwise and scoop out the seeds. Drizzle the cut sides with olive oil and season with salt and pepper.
Place the squash halves cut-side down on a baking sheet lined with parchment paper and roast for about 30-40 minutes, or until the flesh is fork-tender.
While the squash is roasting, prepare the filling. In a large bowl, combine the chopped spinach, artichoke hearts, cream cheese, sour cream, 1/2 cup of mozzarella cheese, Parmesan cheese, minced garlic, oregano, salt, and pepper. Mix until fully combined.
Once the squash is done roasting, remove it from the oven and let it cool slightly.
Using a fork, scrape the flesh of the spaghetti squash to create strings and transfer them to the bowl with the filling mixture. Stir until combined.
Spoon the spinach and artichoke mixture back into the squash halves.
Top each filled squash half with the remaining mozzarella cheese.
Return the stuffed squash to the oven and bake for an additional 15-20 minutes, or until the cheese is melted and bubbly.
Remove from the oven, let it cool for a few minutes, and garnish with chopped fresh parsley before serving.