OpenOffice is fully compatible with all MS Office applications. I've been using it for three years now and just upgraded to their latest version (free open source).
Now be aware that while you can read any MS Office application using OpenOffice, you cannot send a saved OpenOffice file to someone with MS Office.
What you have to do is use the "Save As" function (in OpenOffice) and create a separate version of the file which is formatted for MS Office. You can then use that separate version to share with someone who has MS Office