Put the chopped fish in a wide shallow dish and sprinkle over the oregano, salt and lime juice. Leave this for 8 minutes.
Drain the fish; it will have made a milky liquid. Add the scallions, chile and cilantro and stir gently together.
You can either put teaspoonfuls onto little toasts and sprinkle over some more cilantro, or put the ceviche into a bowl and serve with tortilla chips.