Add 2 tablespoons Old Bay to a large pot of salted boiling water. Add spaghetti and cook according to package directions until al dente. Reserve 1 cup pasta water, then drain
Meanwhile, Heat olive oil a large skillet over medium heat. Add onion, cook until tender, about 5 minutes. Stir in sausage until heated through and begins to brown. Add corn, shrimp, and reserved pasta water; bring to a simmer.
Add cooked spaghetti and green onions; toss until combined.