Preheat oven to 300 degrees. Sprinkle a large roasting pan with olive oil. Lay the salmon in the pan with the skin-side down. Rub a thin slick of oil over the salmon and season with salt. Roast until just cooked through, 20 to 30 minutes.
Meanwhile, dissolve wasabi powder in 8 teaspoons of water. Whisk into sour cream and season to taste with salt. Add more wasabi if desired. Serve with salmon.