Heat vegetable oil in a large skillet or wok over high heat until shimmering.
Add beef strips in a single layer and sear for 1-2 minutes per side until browned; remove beef and set aside.
Reduce heat to medium and add 2 tablespoons of butter to the skillet.
Add minced garlic and sauté for 30 seconds until fragrant.
Add chilled rice to the skillet, using a spatula to break up any clumps.
Drizzle soy sauce and oyster sauce over the rice, then season with black pepper.
Increase heat to medium-high and stir-fry the rice for 3 minutes until lightly toasted.
Return the beef to the skillet and add the remaining 2 tablespoons of butter.
Toss all ingredients together for 1 minute until the butter is melted and the beef is heated through.
Remove from heat and garnish with sliced green onions before serving.