Put the sweet potatoes on a sheet tray and roast for 35 to 40 minutes. The sweet potatoes might still be a little hard in the center- no problem!
While the sweet potatoes are baking make the topping. In a food processor, combine the oatmeal, brown sugar, olive oil, butter, and walnuts and pulse until mixture comes together.
When the potatoes are cool enough to handle, peel them and cut them into 1-inch chunks. Discard the skins. Put the potatoes into a large dish and add the remaining ingredients. Mash together with a potato masher until well combined.
Transfer the mixture to a 2-quart casserole dish. Crumble the topping mixture over the potatoes. Bake until the mixture hot all the way through and the topping is brown and crispy, about 30 minutes.