Chop the apples into 1/4-inch pieces and set them aside.
In a large mixing bowl, combine cream cheese, sugar, and cinnamon. Beat until smooth, then gently fold in the chopped apples.
Slice the croissants down the center to create a pocket, being careful not to pierce through. Spoon approximately 2 tablespoons of the cream cheese mixture into each croissant.
Butter a 12×8 baking dish and arrange the filled croissants facing up.
In another bowl, whisk together milk, eggs, and vanilla extract. Pour this custard mixture over the croissants and set aside.
In a separate bowl, combine flour, oats, brown sugar, salt, and butter. Use a pastry cutter or hand blender to create a crumbly mixture.
Sprinkle the oat crumble over the croissants in the baking dish, cover with aluminum foil, and refrigerate overnight.
When ready to bake, let the French toast sit out for 30 minutes to reach room temperature.
Preheat the oven to 375°F.
With the foil still covering the dish, bake the French toast for 20 minutes. Remove the foil and bake for an additional 10-15 minutes or until the top is golden brown. Serve immediately.