Preheat oven to 400 F.
Pierce spaghetti squash with fork and bake 45 minutes.
Remove from oven and cool. Reduce oven to 350 F.
Cut squash in half and discard seeds. Scoop out pulp, reserve shells. Set both aside.
In a skillet, heat oil and add onion, pepper and garlic.
Saute 5-7 minutes.
Stir in ground meat, sauté until meat is no longer pink.
Add spaghetti sauce, oregano and reserved squash pulp and cook, stir occasionally until liquid is absorbed.
Spoon half of mixture into each shell.
Place shells in baking dish and bake 20 minutes.
Sprinkle cheese on each one and then broil until melted and browned.