Preheat oven to 425°F. Line a large sheet pan with parchment paper or foil and spray with cooking spray.
Place potatoes in a large zip-top plastic bag.
In a small bowl, whisk together 2 teaspoons of Old Bay seasoning and 2 tablespoons of olive oil.
Pour oil mixture into zip-top bag, seal shut, and gently toss to coat potatoes.
Spread seasoned potatoes onto prepared baking sheet.
Bake for 10 minutes, stirring with a spatula halfway through.
Remove pan from oven. Stir potatoes again.
Arrange the corn cobs in a single layer around the potatoes. Brush corn with about 1 tablespoon of melted butter and season with a dusting of Old Bay.
Return the pan to the oven and continue baking for another 10 minutes.
Remove pan from oven. Stir potatoes and corn.
Arrange the sliced sausage and the shrimp around the potatoes and corn on the tray.
Brush shrimp with remaining butter and sprinkle with Old Bay.
Return the pan to the oven and continue baking for another 10 minutes, or until potatoes are crispy and shrimp is pink.
Garnish with sliced green onions just before serving.