In general, it won’t make much of a difference whether you use type O or N mortar in this case. Also, I would stay away from lime mixes unless you in fact know that was what was used before (its also amongst the weakest mortar types).
You are more interested in the components of your mortar than the strength rating (the type O or N in this case), unless your building is something other than house or more than three stories tall.
Limestone can be highly reactive to some components of cement and mortar. If your local hardware stone has mortar for limestone, you can go with that, but typical cements and mortars can change formulas depending on market values.
So, how do you tell?
Get a small sample of the mortar you plan to use. Mix it up and apply to an area of limestone, preferably to a number of stones. Let it dry. Check for any white or chalky material that may have formed on the mortar or around the edges, especially the bottom edges of the mortar. If this happens, you don’t want this mix. If it doesn’t happen, soak the mortar completely again and wait for it to dry. Do not help it to dry by applying heat or fans. Check it again once completely dry. If you still don’t see anything you should be out of danger, but you may want to give one more cycle.
Provided you didn’t get anything, the mortar will at least work for re-pointing. But, remember that you may still get white efflorescence over time. Limestone is very reactive, as I pointed out before. Should this happen (down the line and not during our test) use something like a wire brush. Do not use muriatic acid like they do for bricks. It can dissolve your stones.
I hope that helps.
MKDesign 