The current design of the CMDB is based upon the Atrium data model which (1) uses an inherited class model and (2) currently views the OS as a separate CI. The OS Type field is associated to the Operating System field set (class). It cannot be moved to another class. If you want to identify the OS directly on the computer system record (field set), you will need to either create the associated CI using the Operating System record or add a custom field to the computer system class (field set).
Safe harbor - We have been discussing an option to provide a more direct association (ex. new OS field on the computer system field set) with the option to populate it directly or pull it from the associated OS class record.
I recommend you create an idea to drive a discussion and determine the level of interest. I can drive a more detailed discussion around an acceptable approach to support the range of use cases if interest drives this up the priority list for future consideration. Thanks