Heat wok or heavy skillet over high flame. Add peanut oil and heat until shimmering, but not smoking. Add onion and mushroom; stir fry 30 seconds.
Add chopped cooked chicken, add peas and cook until just heated through, about 1 minute. Stir in oyster sauce, add rice and stir to heat through, about 1 minute.
Make a well in the middle of the rice mixture. Mix eggs and soy sauce and pour into cleared center. Cook and stir eggs, incorporating the rice a small amount at a time. By the time all the rice is mixed in the eggs should be cooked. Sprinkle with sesame oil and toss well. Serve immediately.