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:
<a href=”http://sourceforge.net/projects/odf-converter/files/”>OpenXML/ODF Translator Add-in for Office</a>