Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced onion and cook for about 2 minutes until it begins to soften.
Stir in the ground turkey, breaking it apart with a spatula. Cook until browned and cooked through, about 5-7 minutes.
Add the sliced bell peppers, garlic, and ginger to the skillet. Stir-fry for another 3-4 minutes until the peppers are tender-crisp.
Pour in the soy sauce, oyster sauce (if using), sesame oil, and red pepper flakes. Stir well to combine and cook for an additional 2 minutes.
Season with salt and pepper to taste. Remove from heat.
Serve the stir-fry over cooked rice or noodles and garnish with sliced green onions.