Prepare the oat crumble topping first, so it's ready to go on the muffins as soon as they're mixed. In a small bowl, combine the oats, butter, brown sugar, and cinnamon. Use your hands to massage the oats, brown sugar, and cinnamon, into the butter until it's evenly combined and clumpy. Refrigerate the mixture until you're ready to top the muffins.
Preheat the oven to 500ºF. In a large bowl, combine the all-purpose and whole wheat flours, oats, brown sugar, white sugar, salt, baking powder, and cinnamon. Stir well.
In a separate bowl, whisk together the milk, eggs, and oil.
Pour the whisked wet ingredients into the bowl with the dry ingredients. Stir just until they are combined, it doesn't need to be perfectly mixed. There may still be clumps and a few dry spots, but it's important to avoid over mixing.
Add the frozen berries to the mixture and gently fold in. Again, avoid over mixing because the berry juice will turn the muffins blueish grey.
Line eight muffin tin cups with paper liners, then fill each one to the top with the muffin batter (use more cups if needed, just make sure to fill to the top). Sprinkle the prepared oat crumble topping onto each muffin.
Transfer the muffin tin to the oven and immediately turn the heat down to 400ºF. Bake for 22-25 minutes, or until the muffins have risen into peaks, have cracked slightly, and are deep golden brown on top. Remove the muffins from the oven and the muffin tin (use a knife to loosen any parts that have spilled out over top) and allow them to cool.