It depends on exactly what you want when you say "OS version". Some possibilities:
tsysOS.OSnamee.g. "Win XP", "Win 7", "Win 10"
tblAssets.SPService pack
tblOperatingsystem.Captione.g. "Microsoft Windows 10 Pro", "Microsoft Windows 7 Professional", "Microsoft Windows Server 2012 R2 Standard"
Windows 10 ("no more versions!") version info:
tblAssets.Version
tblAssets.BuildNumber