Whisk egg, buttermilk, and maple syrup in a small bowl. Whisk oat flour, cornmeal, rice flour, buckwheat flour, baking powder, baking soda, and salt in a large bowl.
Whisk buttermilk mixture into dry ingredients, then whisk in butter until no lumps remain.
Heat a large nonstick or cast-iron skillet over medium heat; lightly brush with oil. Working in batches, pour batter by 1/4-cupfuls into skillet. Cook until bottoms are browned and bubbles form on top of griddle cakes, about 3 minutes. Flip and cook until griddle cakes are cooked through, about 2 minutes longer.