Soak the onion slices in one cup of cold water with 1 tsp of salt, rinse and drain well. Marinate with the juice of 1 lime.
Remove the oyster meat from the oysters, saving the oyster juice, and chop the oysters into small cubes.
In a medium sized bowl combine the chopped oysters, the oyster juices, marinated red onion, diced tomatoes, diced bell peppers, lime juice, finely chopped cilantro, oil and salt to taste.
Let marinate in the fridge for about 5-10 minutes and serve immediately.