Given the inclusion of first and last names in the request, office probably refers to tblADUsers.Office.
SELECT Top 1000000
tblAssets.AssetID,
tblAssets.AssetName,
tsysAssetTypes.AssetTypename,
tsysAssetTypes.AssetTypeIcon10 As icon,
tblAssets.IPAddress,
tblAssets.Lastseen,
tblAssets.Lasttried,
tblAssetCustom.Manufacturer,
tblAssetCustom.Model,
tblADusers.Firstname,
tblADusers.Lastname,
tblADusers.Office,
-- short form of OS name
tsysOS.OSname,
-- alternatively, more details
tblOperatingsystem.Caption,
tblOperatingsystem.ServicePackMajorVersion,
tblOperatingsystem.ServicePackMinorVersion,
tblOperatingsystem.Version
FROM
tblAssets
INNER JOIN tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
INNER JOIN tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
LEFT JOIN tblADusers On tblAssets.Username = tblADusers.Username And tblAssets.Userdomain = tblADusers.Userdomain
INNER JOIN tblOperatingsystem On tblAssets.AssetID = tblOperatingsystem.AssetID
INNER JOIN tsysOS On tblAssets.OScode = tsysOS.OScode
WHERE
tblAssetCustom.State = 1
I don't have any Win10 in my inventory yet, so I can't speak to how that shows up. If the short name is all you need, remove tblOperatingsystem from the report. If you want the extra detail, drop tsysOS.