1. Heat the vegetable oil in a large skillet or wok over medium-high heat until shimmering. Add the diced onion and cook for 2-3 minutes until it starts to soften and become translucent. Stir in the minced garlic and cook for another 30 seconds until fragrant, being careful not to burn it.

2. Add the diced cooked chicken to the skillet and stir-fry for 1-2 minutes to heat through. Then, add the frozen mixed vegetables and continue to cook for 2-3 minutes, stirring occasionally, until the vegetables are tender-crisp.

3. Push the chicken and vegetable mixture to one side of the skillet, creating an empty space. Add the cold cooked rice to the empty side and break it up with your spatula. Let the rice cook undisturbed for about 1 minute to get a slight crisp, then stir it into the chicken and vegetable mixture until everything is well combined.

4. Create another clear space in the skillet by pushing the rice mixture to one side. Pour the lightly beaten eggs into the empty space. Scramble the eggs quickly with your spatula until they are just set but still moist. Once cooked, break them into smaller pieces and mix them into the fried rice.

5. Pour the soy sauce, oyster sauce (if using), and sesame oil evenly over the entire mixture in the skillet. Sprinkle with black pepper.

6. Stir everything together thoroughly, ensuring all the rice and ingredients are coated with the sauces. Continue to stir-fry for another 1-2 minutes to allow the flavors to meld and the rice to heat completely. Remove from heat and garnish generously with chopped green onions before serving.