Preheat your oven to 375°F (191°C).
In a mixing bowl, mash together the softened butter, brown sugar, cinnamon, and nutmeg until smooth.
Stir in the oats, along with raisins, cranberries, or nuts if using. Set this filling mixture aside.
Core the apples using a sharp paring knife and spoon. Be careful to leave the bottom intact to hold the filling.
Place the cored apples into a baking dish (8 or 9-inch pan works well).
Spoon the filling into each apple cavity, packing it in until full.
Pour the warm water into the baking dish to keep the apples moist while baking.
Bake for 40–45 minutes, or until the apples are tender and the filling is golden.
Optional: Baste the outside of the apples with the pan juices for added moisture.
Serve the baked apples warm with salted caramel, whipped cream, or a scoop of vanilla ice cream, if desired.