In a large saucepot, melt butter. Add the oats and stir for 2 minutes to toast. Add apples and stir to combine. Add the boiling water and reduce heat to a simmer. Keep at a low simmer for 25 minutes, without stirring.
Add milk, buttermilk, sugar and cinnamon. Stir, gently, to combine. Cook for an additional 10 minutes. Spoon into a serving bowl.
To reheat, place 1 cup cooked oatmeal in microwave proof bowl. Stir in 1/3 cup buttermilk. Microwave in 1 minute increments, stirring between, until hot.
Overnight Variation Using Slow Cooker
Use a 3-1/2 quart or larger slow cooker. Spray the inside of the crock with nonstick spray. Add all ingredients in the crock. Set slow cooker to cook on low for 6 to 7 hours.