Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the ground beef to the skillet, breaking it up with a spatula. Cook until browned, about 5 minutes.
Drain any excess fat from the ground beef, if necessary.
Add the diced onion and minced garlic to the skillet. Sauté for about 2 minutes, until the onion becomes translucent.
Stir in the diced bell pepper and cook for an additional 2 minutes.
Add the chopped cabbage to the skillet, stirring well to combine. Cook for about 3-4 minutes, until the cabbage is tender but still crisp.
Pour in the soy sauce, oyster sauce (if using), and minced ginger. Season with salt and pepper to taste. Stir everything together and cook for another 1-2 minutes.
Remove from heat and drizzle with sesame oil, if desired.
Serve the stir fry over cooked rice or noodles.