Preheat oven to 350°F. and line an 8x8-inch baking dish with parchment paper. Set aside.
Using a hand mixer or stand mixer, beat together first amounts of coconut oil, coconut sugar, oat milk and the eggs, vanilla, and lemon zest in a large bowl until smooth and creamy.
Add the baking powder, baking soda, and sea salt and blend until smooth. Stir in the first amount of flour until just combined. Fold in the blueberries with a spatula. Pour the batter into the prepared baking dish.
Prepare the streusel topping: whisk together the second amounts of flour, coconut oil and coconut sugar until crumbly. Use your hands to break the streusel into small pieces and sprinkle over the top of the batter.
Bake coffee cake for 70 to 80 minutes, or until toothpick inserted into the center comes out clean. Let coffee cake cool to room temperature.
Prepare glaze: stir together powdered sugar and second amount of oat milk (Add another tablespoon or two of oat milk, if needed, to reach desired glaze consistency). Drizzle over coffee cake before serving and enjoy!