Soak mushrooms in warm water to cover until softened, about 20 minutes. Drain, discard stems and finely chop caps.
Crumble meat into a large bowl. Add mushrooms, onion, water chestnuts, green onions, garlic, soy sauce, oyster-flavored sauce, and sesame oil. Mix well. Sprinkle cornstarch over meat mixture & mix well. Divide mixture into 4 equal portions into a patty about 1" thick.
Place patties on a oiled grill 3-4" above a solid bed of low-glowing coals or on a medium-high gas grill. Cook, turning once, until done to your liking, 3-4 minutes per side for medium-rare.