Do I have a fieldstone foundation and if so, do I need to use Type O mortar?
Type “O” mortar is often used because it is not very strong and it’s somewhat flexible. We generally use it for “sandstone” not fieldstone and interior use only. See below:
https://www.builddirect.com/learning-center/flooring/different-types-of-mortar-and-their-uses/
I’d use Type N for exterior use.