The orange flavor in these O comes from zest in the batter and two brushings of fresh juice once they're baked. A beautiful buttercream poppy sits atop each one, hinting at the seeds within.
Preheat oven to 350 degrees. Line 20 cups of two 12-cup muffin tins with baking cups. Sift flour, baking powder, baking soda, and 1/2 teaspoon salt into a bowl. Stir together milk and 1/4 cup orange juice.
Beat butter, 1 1/4 cups sugar, and the orange zest with a mixer on medium speed until pale and fluffy, about 4 minutes. Beat in eggs, 1 at a time. Reduce speed to low, and add flour mixture in 3 additions, alternating with milk mixture, beginning and ending with flour. Beat in poppy seeds.
Divide batter among muffin tins (filling each cup two-thirds full). Bake until golden around the edges, 18 to 20 minutes.
Meanwhile, bring remaining 1/4 cup orange juice and remaining 1/4 cup sugar to a boil in a small saucepan over medium heat, stirring until sugar dissolves. Remove from heat. Let stand until ready to use.
Brush warm cupcakes with half the orange juice mixture. Transfer cupcakes to a wire rack, and let cool for 30 minutes.
Brush cupcakes again with remaining orange juice mixture, and let cool completely.
Frost cupcakes with untinted buttercream. Top each with a buttercream poppy.
Unfrosted cupcakes can be stored at room temperature for up to 2 days.