Line the base of an 18cm x 28cm slice pan. Process flour, oats and 1 teaspoon salt in a food processor until oats are finely ground (I did this in a blender because I don't have a food processor).
Using an electric mixer, beat butter, sugar and orange zest until light and fluffy. Gradually beat in flour mixture until shortbread mixture comes together in small clumps.
Press just more than half of the shortbread mixture over the base of the prepared pan, then, using a spatula, level and smooth the top. Spread over marmalade, then scatter with chocolate. Crumble over remaining shortbread mixture, then refrigerate for 30 minutes.
Meanwhile, preheat oven to 150C. Bake shortbread for 45 minutes or until light golden. Cool in pan.
Cut into 24 slices and serve. The shortbread will keep in an airtight container at room temperature for up to 3 days.