Cook lo mein noodles according to package instructions, then drain and set aside.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the sliced flank steak and cook until browned, about 3-4 minutes. Remove steak and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Sauté the mixed vegetables and garlic until tender, about 5 minutes.
Return the cooked steak to the skillet and mix in the cooked noodles.
In a small bowl, whisk together soy sauce, oyster sauce, sesame oil, sugar, and black pepper. Pour the sauce mixture over the noodles and stir well to combine.
Cook for an additional 2-3 minutes, allowing the flavors to blend. Serve hot.