Try this:
$OU = "OU=User Accounts,OU=Accounts,OU=Common,OU=GB,OU=Users,OU=test,OU=OMEGA,DC=corp,DC=domainname-group,DC=com"
$CSVname = "C:\Users\user\Documents\outputresult.csv"
Get-ADUser -SearchBase $OU -Filter * -Properties SamAccountName,GivenName,Surname,EmailAddress,Department,Manager,Office |
Select-Object SamAccountName,GivenName,Surname,EmailAddress,Department,@{n='Manager';e={(Get-ADUser $_.Manager).saMAccount}},Office |
Export-Csv $csvname