• Adjust oven rack to the lower middle position. Preheat oven to 350F.
• Butter a 9-inch cake pan or springform pan. Dust pan with flour, shaking out the excess.

Make the Topping

• In the bowl of a food processor, process flour, oats, brown sugar, cinnamon, and salt until combined, a few short pulses.
• Add butter and process until mixture resembles wet sand, about 20 1-second pulses.
• Refrigerate until needed.

Make the Cake

• Whisk together the flour, baking powder, and salt in a medium bowl. Set aside.
• In a standing mixer with a flat beater, cream the butter and sugar on medium speed until light, about 3 minutes.
• Switch the mixer to medium low and add the orange zest and vanilla.
• Then add the eggs, one at a time, until well mixed, scraping down the sides of the bowl with a spatula as needed. The mixture will look 'curdled' at this point.
• Reduce the speed to low and slowly add the flour mixture. Mix until it is nearly combined, but you still have a few streaks of flour. Shut off the mixture and give the batter a few swift strokes with the spatula to finish mixing.
• Add the peaches and fold them in.

Bake the Cake

• Pour the batter into the prepared cake pan, smoothing it to the edges. Sprinkle the topping evenly over the batter, breaking up the clumps with your fingers.
• Bake for 45-50 minutes, or until a toothpick comes out with some crumbs attached. You don't want the cake to be too dry.
• Cool on a wire rack for 30 minutes and then serve.