Preheat the oven to 350 degrees F. Grease a 9x5 inch loaf pan.Cream the butter and sugar in a large mixing bowl until fluffy. Add the eggs, hazelnut oil, and vanilla extract and beat until combined. Add the orange zest and buttermilk and beat until combined.
In a separate bowl, sift together the flour, baking powder, baking soda, and salt. Add the flour mixture to the wet mixture and stir with a rubber spatula just until combined. Stir in the ground hazelnuts. Prepare the swirl mixture by combining all ingredients.
Pour 2/3 of the batter into the loaf pan. Sprinkle evenly with 1/2 of the swirl mixture. Pour the remaining batter over the top and spread with a rubber spatula to even it out. Sprinkle with the remaining swirl mixture and gently press it into the batter.Bake for 45-55 minutes or until a toothpick inserted into the center of the loaf comes out clean but moist. Be careful not to over-bake or the bread will be dry.Let the bread cool in the pan for at least 15 minutes, then transfer to a wire rack to cool.Slice and serve.