Heat grill. In large bowl, combine chicken, onion, oregano, salt and pepper; mix well. Shape mixture into four 4-inch-long oval patties.
When ready to grill, carefully oil grill rack. Place patties on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals. Cook 13 to 15 minutes or until thoroughly cooked and no longer pink in center, turning once.
Spread inside of each pita bread half with 1 tablespoon hummus. Line each with cucumber and tomato slices. Add chicken patty to each.