Place a sieve over a bowl, add rice, and rinse it with water. Repeat this process several times, changing the water each time.
Soak the rinsed rice in water in the bowl for at least 30 minutes.
Add sake and salt to a small pot and bring it to a boil. Once it starts to boil, reduce the heat to medium and add oysters. Let them simmer for a few minutes, flipping them halfway through.
Remove the oysters from the pot. Chop half of them and leave the other half whole. Do not discard the cooking liquid, as it will be used later.
Peel ginger and slice it into thin strips. Thinly slice green onion.
Transfer the cooking liquid from the pot to a measuring cup. Add mirin and light soy sauce, then pour in kombu dashi (or water, if kombu dashi is unavailable) until the total volume reaches ⅚ cup (200 ml) for 2 servings. For 4 servings, use 1 ⅔ cups (400 ml), and for 6 servings, use 2 ½ cups (600 ml).
Drain the water used to soak the rice. Add the drained rice along with the seasoned liquid from the measuring cup to a large pot. Then, evenly distribute the chopped oysters on top.
Cover the pot and bring it to a boil. Once boiling, reduce the heat to low and continue cooking for 10 minutes, or slightly longer for larger quantities (11 minutes for 4 servings, 12 minutes for 6 servings).
Turn off the heat. Remove the lid and arrange the remaining whole oysters and ginger on top. Cover the pot again and let it steam for 10 minutes. (If you are using a rice cooker instead of a pot, add the oysters after the rice has cooked, close the lid, and let them steam for about 5 minutes.)
Serve in individual bowls and garnish with the green onions.