Cut a thin slice from stem end of each tomato and scoop out and discard seeds with a small spoon. Carefully scoop out pulp to form a tomato shell, then finely chop pulp.
Drizzle inside of each tomato shell with oil and lime juice and let stand at room temperature while making guacamole.
Pit and peel avocados and coarsely mash with a fork in a bowl. Fold in chopped tomato pulp, onion, cilantro, chile, and salt, then spoon into tomato shells, mounding guacamole.