Do a search for the following file: "TNSNAMES.ORA"
This file keeps connection information for your Oracle databases. You will probably have several versions of this on your computer.
Sometimes you can have this problem if the entries are not in the right order, or there are conflicting versions of the file. You will find more than one of these files on these computers.
The users probably did some updating or have connected to different Oracle databases in the past.
What you may want to do, after backing up these files, or course, is to open them with Notepad, compare entries, and consolidate the files so that all source names are in the same file. Then mess around with the order of the entries, moving your Excel apps database further up the list.
When you are done, copy the file to all the locations where you found it. If you have more than one ORA directory, then this could definitely be a problem.
Be careful with the syntax and just be sure to copy it so that you don't leave out a character.
Anyway, it doesn't sound like a DSN issue.
Maybe this will help.