We are often asked for OS2 fibre in the belief that’s it’s superior to OS1 (as OM4 is to OM3). But is this true and are manufacturers claims of their cable conformance correct? In many cases, it seems, the answer is no.
Below is a useful link to a more detailed explanation from the FIA.
The easiest points to remember are OS1 is for Indoor / Tight Buffered cable; OS2 is for Outdoor / Loose Tube cable. I believe the confusion has arisen as OS2 grade fibre (as in 250um) has been used in the former, but once the 900um tight buffer is applied and it’s manufactured into a cable and stresses have been applied, it no longer meets the OS2 classification.