Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the diced onion and sauté until translucent, about 3-4 minutes.
Add the minced garlic and stir-fry for another minute.
Add the mixed frozen vegetables and cook until they're tender, following the package instructions.
Push the vegetables to one side of the pan. Crack in the eggs and scramble them until cooked through.
Mix the eggs with the vegetables.
Add the cooked rice to the pan, breaking up any clumps with a spatula. Stir-fry everything together for about 5 minutes, ensuring the rice is heated through and starting to brown.
Add the soy sauce, oyster sauce (if using), salt, and pepper. Taste and adjust the seasoning as needed.
Transfer the fried rice to a serving platter and garnish with chopped scallions (if using). Serve immediately.