Cut spaghetti Squash in half lengthwise, scoop out seeds. Place cut side down on a microwave safe dish. Microwave for 12-15 minutes, or until flesh is easy pierced with a knife.
When cool enough to handle, use a fork to shred the spaghetti squash into noodle shapes.
Wrap spaghetti squash flesh in a towel, squeeze out as much excess moisture as possible. Set aside.
Meanwhile, heat olive oil in a large sauté pan over medium heat.
When hot, add onions. Saute until starting to soften, about 5 minutes.
Add sausage and cook until browned. Drain excess fat.
Stir in garlic and cook for about 30 seconds, until fragrant. Add tomato sauce, salt, pepper and oregano. Stir to combine. Turn heat to low and simmer for about 10 minutes.
Remove about 1 cup of sauce from pan, set aside.
Add spaghetti squash and stir to mix into the sauce.
Transfer spaghetti squash mixture to a large casserole pan.
Top with the reserved sauce, then top with the shredded cheese.
Bake in the oven for 10-15 minutes, or until cheese is melted and bubbly. Finish under broiler for 1-2 minutes to get cheese a little browned if desired.