OpenOffice can open Excel files, but AFAIK, Excel cannot open OO files.
Have you done that before?
We have some OO users, and what they do to avoid these problems is saving their files in MS Office format (.xls, .doc, etc).
Apparently, there is an office Add-in you can download, but I have never tried it and I cannot recommend it.
Here’s the link in case you want to give it a try: OpenXML/ODF Translator Add-in for Office.