Mix sugar, soy sauce, and oyster sauce together in a resealable gallon-sized bag. Add Spam and seal the bag; turn the bag, gently squeezing to coat Spam. Marinate for at least 1 hour.
Bring water to a boil; add rice. Return to a boil, then reduce the heat to low and simmer until liquid is absorbed and rice is tender, about 15 minutes. Stir vinegar into rice until combined.
When the rice is almost finished cooking, heat a skillet over medium-high heat. Add Spam and marinade. Cook, stirring frequently, until Spam is browned and marinade is absorbed, about 5 minutes.
Stir rice and seaweed together in a bowl; add luncheon meat and mix well.
