Break up the cold rice with your hands so there are no large clumps.
Heat a large wok or skillet over high heat until smoking, then add the oil.
Add the butter and let it melt, then add the onion, garlic, and carrots; stir-fry for 1-2 minutes.
Push the vegetables to the side and pour the beaten eggs into the empty space; scramble until just set.
Add the cold rice to the wok and toss vigorously to combine with the vegetables and eggs.
Pour the soy sauce and oyster sauce (if using) evenly over the rice; toss continuously to coat.
Add the thawed peas and stir-fry for another 1-2 minutes until heated through.
Season with salt and a generous pinch of white pepper.
Remove from heat, garnish with green onions, and serve hot.