Preheat oven to 350 degrees F. Coat an 11-by-7-inch baking dish with cooking spray. Place sweet potatoes in a large saucepan and add water to cover; bring to a boil over high heat. Reduce heat to medium to maintain a simmer; cook until the sweet potatoes are tender, about 16 minutes. Drain well. Place the sweet potatoes in a large bowl.
Meanwhile, combine oats, pecans, 5 tablespoons brown sugar and ¼ teaspoon salt in a bowl. Stir in butter. Set aside.
Add milk, eggs, cinnamon and the remaining 3 tablespoons brown sugar and ¾ teaspoon salt to the drained sweet potatoes. Beat with an electric mixer fitted with a whisk attachment on medium speed until smooth, about 2 minutes.
Spoon the sweet potato mixture into the prepared baking dish; sprinkle evenly with the oat mixture. Bake until golden and bubbly around the edges, 30 to 40 minutes.
