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.
- Perform a Custom filter for Organization Unit.
- Click on the Export button
- Select CSV file extension
Ref: https://docs.microsoft.com/en-us/answers/questions/239938/export-ad-users-to-csv-file.html