File do not have "users"? Files have "owners".
Since we do 't write custom scripts I can only say that you need to learn how to retrieve the file owner Which is part of the FileInfo structure. Then test if that owner
Example:
$f = Get-Item test.txt
$ownerID = $f.GetAccessControl().Owner
Then check that account against your OU by getting the distinguished name and matchi g it to the OU.
Look in the Gallery for more example scripts.
\_(ツ)_/