Heat 2 quarts salted water (1 teaspoon salt) to boiling; reduce heat.
Simmer until opaque, about 1 minute.
Remove with slotted spoon and drain; reserve.
Place squid in same water.
Cover and cook until tender, about 30 minutes; drain.
Shake oil, lemon juice, parsley, garlic, oregano and salt in tightly covered container.
Toss with scallops, squid, onion and olives.
Cover and refrigerate, stirring occasionally, at least 4 hours.
Serve on lettuce leaves with lemon.