If you’d like to keep the pancakes warm between batches, place a rack in the center of your oven and preheat the oven to 200 degrees Fahrenheit.
In a small microwave-safe bowl, microwave the butter for 15 seconds. Continue to microwave in 15-second bursts, just until melted. Set aside and let cool to room temperature.
Place the Greek yogurt, milk, oats, eggs, maple syrup, vanilla, cinnamon, baking powder, salt, and melted, cooled butter in a blender. Blend until the batter is very smooth, stopping to scrape down the sides of your blender a few times. Depending on your blender, this will take a few minutes. Continue blending until you don’t see any remaining bits of oats, and the batter is creamy.
Stop the blender. Peel and grate the apple, then stir it into the blender. Do not blend again.
Heat a large nonstick skillet or griddle over medium-low heat. Unless the manufacturer of your pan directs otherwise, lightly coat the surface of the pan with nonstick spray (some nonstick pans and griddles may not need it).
Once the skillet is heated, drop the batter by 1/4 cupfuls onto the pan and, with the flat bottom of your measuring cup or the back of a spoon, spread the pancakes to be 1/2 inch thick. Let cook slowly for 3 to 4 minutes on the first side, until the edges look dry and small bubbles appear on top. Gently flip (the underside should be golden) and continue cooking on the other side for 1 to 2 additional minutes, until golden. Adjust the heat as needed between batches. If you pancakes are turning golden before they are cooked through, reduce the heat of your skillet accordingly; if they don’t seem to be cooking through at all and you’ve gone a few minutes over the cook time, increase the temperature a little. Repeat with the remaining batter.
Serve the pancakes immediately or place on a baking sheet and keep warm in the oven. Add your favorite toppings and enjoy.