Place tomatoes on a double thickness of heavy-duty foil (about 24 in. x 12 in.). In a small skillet, saute garlic and oregano in butter for 2 minutes. Pour over tomatoes. Fold foil around tomatoes and seal tightly.
Grill, covered, over medium heat for 4-5 minutes on each side or until tomatoes are heated through. Open foil carefully to allow steam to escape.