Sauté onion in oil in a heavy 16-quart stockpot until golden. Add remaining ingredients to pot in order given; cover tightly.
Simmer over medium-high heat for 15 minutes, just until steam begins to escape; add corn. Lower heat to medium; simmer an additional 15 minutes.
Lobsters should be cooked, potatoes and corn tender and shellfish open.
Use a slotted spoon to arrange everything on a platter; top with lobsters cut into serving-size pieces. Season hot broth to taste.