Soak uncooked rice for 4 hours; drain and rinse.
In a saucepan bring 2 cups water to a boil. Add rice and stir. Reduce heat, cover, and simmer for 20 minutes. Stir in rice vinegar, and set aside to cool.
In a separate bowl, stir together soy sauce, oyster sauce, and sugar until sugar is completely dissolved. Slice luncheon meat lengthwise into 10 slices, or to desired thickness, and marinate in sauce for 5 minutes.
In a large skillet, heat oil over medium high heat. Cook slices for 2 minutes per side, or until lightly browned. Cut nori sheets in half and lay on a flat work surface. Place a rice press in the center of the sheet, and press rice tightly inside. Top with a slice of luncheon meat, and remove press. Wrap nori around rice mold, sealing edges with a small amount of water. (Rice may also be formed by hand in the shape of the meat slices, 1 inch thick.) Musubi may be served warm or chilled.