Rinse and trim vegetables, if necessary. Place pattypan squash and zucchini in a large bowl; set aside. Place peas and green onion in a medium bowl; set aside.
In a small bowl whisk together vinegar, oil, oregano, salt, and pepper. Divide the vinegar mixture between the squash mixture and the pea mixture; toss each mixture to coat.
Place zucchini and squash mixture in a grill basket or wok and place on the rack of an uncovered grill directly over medium coals for 5 minutes, stirring occasionally. Add the peas and green onions; grill for 7 to 9 minutes more or until vegetables are crisp-tender, stirring occasionally. (For a gas grill, preheat grill. Reduce heat to medium. Place basket on grill rack. Cover and grill as above.) Makes 4 to 6 servings.