Cook spaghetti according to package instructions, drain and set aside.
Heat vegetable oil in a large pan or wok over medium heat.
Add onion, garlic, and bell pepper. Stir-fry for 2-3 minutes until softened.
Add the mixed seafood and cook for 2-3 minutes until cooked through.
In a small bowl, combine soy sauce, oyster sauce, sugar, and sesame oil. Mix well.
Pour the sauce mixture into the pan with the seafood and vegetables. Stir-fry for 1-2 minutes until well coated.
Add cooked spaghetti to the pan and toss to combine with the sauce and vegetables.
Garnish with chopped green onions and sesame seeds.
Serve hot and enjoy!