In new construction, most experts do not recommend Type O mortar in freeze-thaw environments. This is because Type O mortars can deteriorate after relatively few freeze-thaw cycles. When repointing existing masonry, however, it is extremely important to use a mortar that is softer than the original. Harder mortar can cause cracking or spalling of the brick units, which is far more serious than deterioration of the mortar.
In the appendix for ASTM C 270, Type O mortar is listed as an alternative to Type N for areas exposed to freezing. When properly installed and tooled, repointed mortar joints are often better compacted than the original mortar since the mortar is placed in thin layers that are compacted separately. The resulting joints are tight and well filled and, therefore, allow very little water penetration. With lower water-penetration rates, they will perform reasonably well in a freeze-thaw environment.