Preheat the oven to 320°F/160°C.
Generously grease and flour the pan(s) and set aside.
Place chocolate in a microwave-safe bowl and microwave in 20-30 second intervals, stirring well until the chocolate is completely melted and smooth. Stir in one tablespoon of oil.
Using an electric mixer, whisk together eggs, sugar, and vanilla extract on high speed for 10 minutes until light and fluffy.
Reduce the speed to low and gradually add (in this order!) oil, water, orange juice, flour, and baking powder.
Pour ¾ of the batter into the prepared pan(s).
Stir the melted chocolate with the remaining batter until thoroughly combined.
Place tablespoons of the chocolate mixture over the white mixture in the pan. Using a knife or a skewer, swirl the chocolate batter into the white cake batter to create a marbled effect.
Bake for 45-50 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (Do not over-bake, or the cake will be dry; you want a few crumbs on the toothpick).
Let the cake cool before slicing.
Store cake in an airtight container at room temperature for up to 3 days or in the refrigerator for up to 5 days.The cake can be frozen as a whole or sliced for up to 3 months. After completely cooling, wrap it with aluminum foil or plastic freezer wrap. Thaw at room temperature before serving.