Combine flour, oats, brown sugar, cinnamon, baking soda, and salt in a large bowl. Mix milk and lemon juice in another bowl; stir in banana. Stir banana mixture into flour mixture; fold in rhubarb.
Heat a griddle or large skillet over medium heat. Drop 1/4 cupfuls of batter onto the griddle and cook until bubbles form and the edges are dry, about 2 minutes. Flip, and cook until browned on the other side, about 2 minutes more. Repeat with remaining batter.