The below PowerShell command should work
Get-ADUser -Filter * -SearchBase "OU=Research,OU=Users,DC=ad,DC=contoso,DC=com" -Properties * | Select-Object name | export-csv -path c:\temp\userexport.csv
You can also do the same task using AD GUI
- Open AD, Click on Filter Button.
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- Perform a Custom filter for Organization Unit.
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- Click on the Export button
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- Select CSV file extension
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Ref: https://docs.microsoft.com/en-us/answers/questions/239938/export-ad-users-to-csv-file.html