Whisk together salt, baking soda, and flour in a large bowl.
In a medium bowl, whisk together all the sugar and eggs. Whisk in orange zest, vanilla extract, oil, and buttermilk.
Add the wet ingredients to the dry ingredients and whisk until combined.
Fold in cranberries and nuts (if using).
Put the batter into the loaf pan you prepared. Top with chilled streusel, pressing gently.
Bake for 50-60 minutes, covering loosely with foil halfway through to prevent over-browning. If you insert a toothpick, it should come out clean.
Cool completely in a pan on a rack.