Preheat oven to 325°F (160°C).

Mix brown sugar, margarine, and buttermilk in a large bowl.

In a separate bowl, whisk together flour, soda, and salt.

Add the dry ingredients to the wet ingredients and mix until combined.

Add oil, eggs, sugar, and vanilla to the batter and mix well.

Fold in diced apples and nuts.

Pour the batter into a greased and floured cake pan.

Bake for 1 1/2 hours, or until a wooden skewer inserted into the center comes out clean.

Let the cake cool in the pan for 10 minutes before transferring to a wire rack to cool completely.