Cook the udon noodles according to package instructions. Drain and set aside.
In a large skillet or wok, heat the vegetable oil over medium-high heat.
Add the garlic and ginger, sautéing for about 30 seconds until fragrant.
Add the sliced bell peppers, carrots, and snap peas to the skillet. Stir-fry for 3-4 minutes until the vegetables are tender-crisp.
Push the vegetables to the side of the skillet and add the cooked udon noodles.
Pour the soy sauce, oyster sauce, and sesame oil over the noodles. Toss everything together until well combined and heated through, about 2-3 minutes.
Season with salt and pepper to taste.
Remove from heat and garnish with chopped green onions and sesame seeds if desired.