Heat the oil in a large pot with a lid over medium heat. Cook the onion and garlic in the hot oil until fragrant, 3 to 5 minutes. Add the ginger to the mixture and cook another 2 minutes. Stir the oyster sauce into the mixture; cover and cook another 2 minutes. Pour the water into the mixture, cover, and cook another 2 minutes. Add the clams; cover and cook until the majority of the clams have opened, about 5 minutes. Discard any clams which do not open. Serve immediately.