Prep the pan and oven. Heat the oven to 350°F (175°C). Line a 9x5-inch loaf pan with parchment so it overhangs the long sides for easy lifting.
Lightly grease the exposed ends.
Whisk dry ingredients. In a large bowl, whisk oat flour, baking powder, baking soda, salt, and poppy seeds until well combined.
Mix the wet ingredients. In a medium bowl, whisk eggs, yogurt, milk, oil, sugar, honey or maple, lemon zest, lemon juice, and vanilla until smooth.
Combine gently. Pour the wet mixture into the dry and whisk or stir just until no dry pockets remain. The batter will be thick but pourable. Avoid overmixing.
Fill and smooth. Scrape the batter into the prepared pan and smooth the top with a spatula.
Bake. Bake for 45–55 minutes, until the top is golden and a toothpick comes out clean or with a few moist crumbs.
If browning too quickly, tent loosely with foil for the last 10 minutes.
Cool. Let the loaf cool in the pan for 10 minutes, then lift it out and cool completely on a wire rack.
Optional glaze. Stir powdered sugar with lemon juice to a pourable glaze. Drizzle over the cooled loaf and let it set before slicing.