Type O mortar is typically not recommended for masonry walls in freeze/thaw environments. For tuckpointing, however, some architects specify Type O mortar. Won't this mortar disintegrate?
In new construction, most references do not recommend using Type O mortar in freeze/thaw environments. This is because Type O mortars can deteriorate and break down after a relatively small number of freezing and thawing cycles. When repointing existing masonry, however, it is extremely important to use a mortar that is not harder than the original mortar. Harder mortar can cause cracking or spalling of the brick units, which is a far greater problem than mortar deterioration.
In the appendix for ASTM C 270, Type O mortar is listed as an alternative to Type N mortar for areas exposed to freezing. When properly installed and tooled, mortar joints in repointed masonry walls are typically better compacted than the original masonry. The mortar is placed in thin layers that are each compacted. Because of this, the joints are tight and well-filled and therefore allow very little water penetration.