Place potatoes in large pan of salted boiling water; return to boil. Reduce heat to medium; simmer 15-20 minutes or until fork tender. Drain; return potatoes to pan. Toss over medium heat 30 seconds to remove any excess moisture. Remove from heat, cover to keep warm.
Preheat oven to 425 degrees. Spray shallow 2-quart baking dish with cooking spray. Bring half-and-half, 3 TBS butter, salt and pepper to a simmer in a small saucepan over medium heat.
With potato masher, mash potatoes until almost smooth. Pour in half-and-half mixture, mash until smooth and fluffy. Stir in green onions and oregano. Spoon into baking dish. Bake 20 minutes or until potatoes are slightly puffed and hot in center nad crumb topping is golden brown.