Adjust oven rack to middle position and heat oven to 275 degrees. Measure 4 cups of oats in a large bowl; place remaining 1 cup in a blender or food processor canister and grind to a meal texture. Add ground oats to the large bowl, along with peanuts. Meanwhile, bring syrup, molasses, oil, water, and salt to a simmer in a small saucepan over medium-low heat. Remove from heat and add vanilla. Pour liquid mixture over oat mixture; stir to combine.