Snap the pasta in half for more manageable lengths. Fill a 12-inch high-sided skillet about half full with salted water. Bring to boiling. Add spaghetti. You should have just enough water to cover. Cover the skillet and cook 2 minutes less time than given on the package, stirring occasionally. Drain, reserving 1 cup pasta cooking water.
In the same skillet, melt butter over medium heat. Add garlic and cook just until fragrant and softened, about 1 minute. Add oyster sauce, maggi seasoning, and fish sauce. Stir to combine.
Add spaghetti and Parmesan cheese to the skillet. Increase heat to high. Cook, tossing, until mixture is creamy and emulsified, adding pasta water as needed. Top with green onions.
