Cook the noodles according to package instructions, drain, and set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced onions and bell pepper, sautéing for about 3-4 minutes until softened.
Stir in the minced garlic and mixed vegetables, cooking for an additional 2 minutes.
Add the cooked noodles to the skillet, tossing them with the vegetables.
Pour in the soy sauce, sesame oil, and oyster sauce, if using, tossing everything together.
Cook for an additional 2-3 minutes, allowing everything to heat through and meld together.
Serve hot, garnished with chopped green onions.