2 tsp orange zest = how much OJ?
I don't feel like running to the store to get an orange for my orange-spice pumpkin bread. Can I substitute OJ, and if so, how much?
In general, not easily. Orange juice will change the amount of liquid in the recipe, and increase the acidity, which will throw off the chemistry of the baking. I would probably just leave the orange out completely, and make pumpkin spice bread.
1/2 tsp orange EXTRACT or 1/4 tsp lemon juice.
You are going for the tartness of zest, not liquid or sweetness.
Me, three -- orange zest is used as a spice because of the essential oils found only in the skin (and not in the juice) -- juice not only doesn't have those oils, but will water down the batter.
Just leave it out this time -- your bread will be good anyway, I'm sure.
(and when you DO buy zest, make sure you keep it in the fridge so the oils don't evaporate.)