Heat the vegetable oil in a large pan or wok over medium-high heat.
Add the chopped onion and minced garlic. Stir and cook for about 2 minutes until fragrant.
Add the diced chicken breast. Cook for 5–7 minutes, stirring often, until the chicken is fully cooked.
Push the chicken to one side of the pan. Crack the eggs into the empty side and scramble them until fully cooked.
Add the mixed vegetables (carrots, peas, and corn). Stir everything together and cook for 2–3 minutes until the vegetables are heated through.
Add the cooked rice to the pan. Stir well to combine the rice with the chicken, eggs, and vegetables.
Pour in the soy sauce and oyster sauce. Mix everything well so the rice is evenly coated with the sauces.
Season with salt and black pepper to taste. Stir and cook for another 2–3 minutes until everything is hot.
Sprinkle the chopped green onions over the fried rice. Stir gently to combine.