Preheat oven to 350℉/180℃. Grease and flour a bundt pan, tapping out any excess flour.
Peel 1½ large oranges (with a vegetable peeler) and add it to a food processor or blender with the sugar. Pulse until the peels are completely broken down and the sugar turns orange.
In a large bowl, mix together the flour, baking powder and salt until combined and set aside.
Transfer the sugar to the bowl of a stand mixer and add the eggs, oil, and vanilla extract and beat on medium low until combined.
Alternating dry and wet ingredients with the mixer on low, add in half of the flour mixture, all of the sour cream, then the rest of flour mixture just until it comes together. Add in the orange juice and poppy seeds and mix until just combined.
Pour the batter into the prepared pan and bake for 40-50 minutes until the cake is set and a toothpick inserted into the center comes out clean with a few moist crumbs. Transfer the cake to a wire rack for 15 minutes to cool.
While the cake is baking, prepare the simple syrup by bringing all the sugar, orange juice, and water over medium heat until the sugar dissolves.
If making Bundt Cake, turn out the cake onto a serving platter and brush the simple syrup all over the top (previously the bottom). If making a loaf cake, don't turn out the loaf, simply brush it on top.
Once the loaf is completely cool, make the glaze by combining the powdered sugar, orange zest, and orange juice in a bowl and mix with a whisk until smooth. Pour the glaze on top of the cake, then sprinkle with more orange zest.