It isn't an annotation. It is data pulled from 3 data sources - VMware, HDS HiCommand (and the HP rebrand) and ECC. However, it is only used if "SRM" is the method used to identify hosts. VMware is an exception - that datasource automagically identifies and discovers hosts, so VMware hosts always have their operating system.
If you have hds/ecc, and SANscreen was able to get operating system for a WWN , but that host was identified via zone names in auto host resolution, SANscreen will not display the OS.
I am lobbying for an architectural change that would allow us to have OS data be independent of the AHR method used to identify that host. I'd like our file system data utilization datasources to be able to report it
Host OS is not a default annotation. The OS field you are seeing is a data source field for the ECC data source. You would need to create a custom annotation for OS if you wish to annotate OS type directly. This annotation could then be made accessible to the Inventory data mart.