Preheat your oven to 350° F.
Lightly grease a 9” x 5” loaf pan and line with parchment paper, letting some of the paper overhang on at least one side. Greasing and lining may seem like overkill, but it really makes for easy removal, and will prevent the cake from sticking to the bottom of the pan.
Peel and core the apples, then chop into 3/4” chunks. Toss the chopped apples with sugar and spices and set aside. The sugar will draw out some of the moisture, making the apples slightly syrupy.
In a large mixing bowl, combine the dry ingredients of flour, baking powder, sugar, cinnamon, and salt. Mix well so everything is combined.
Make a well in the center of the ingredients, adding oat milk, oil, and vanilla. Stir until the batter is smooth.
To make the streusel crumble topping, combine the flour, brown sugar, and cinnamon in a medium bowl. Stir to combine the flour mixture. Add the water and oil. Crumble the mixture with your fingers into small clumps.
When preparing the cake for baking, add a little more than one third of the batter into the prepared pan, making sure it's level. We want to begin with a good base.
Add half of the apples over top of the batter in an even layer. Add about half of the remaining batter and spread so the apples are completely covered.
Add the final apple layer (reserving a few apple pieces to go under the streusel) and cover with the rest of the batter.
Top the batter with a handful of reserved apples and then cover everything with a layer of streusel crumb topping. Try to make sure the streusel is evenly spread.
Bake the cake for 50-60 minutes, taking into consideration that oven times vary. If your oven runs hot, check the cake a little earlier. The top should be golden brown on the edges, and the middle should be set. Check with a skewer to make sure it comes out clean.
The cake should cool in the pan for at least 20 minutes before moving it to a cooling rack to cool completely. Serve the cake warm or at room temperature.