In a stainless steel bowl combine the fish, lime juice and onion, you will need enough juice to completely cover the fish. Cover and refrigerate for 4 hours until the fish no longer looks raw when broken open. Pour the fish into a colander and drain off the lime juice.
In a large bowl, mix together the tomatoes, green chilies, cilantro, olives and olive oil. Stir in the fish and orange juice and season with salt. Just before serving stir in the avocado, being careful not to break up the pieces.