Before we moved in: there were two separate wall appliance holes. Presumably one for an ancient microwave or something. N (with some help) cut the divider out and made it one big hole for our double oven.
Our double oven fit - but wasn't the exact dimensions of the hole - see the hole across the top and the ragged edges on the sides.
Eventually, we developed a plan. It's not the perfect plan, but it's better than a hole! We spent some time in the trim/molding section at Home Depot, mostly considering things like the dimensions of what we were trying to cover up. The side pieces are something called "shoe" molding, and the top is just crown molding. We originally thought we'd just do the top to the edges of the hole, but the edges were not super clean, and we decided this looks better. We stained them to the approximate color of our cabinets, and boom! No hole. I love it.