I've never had any compatibility problems with OpenOffice files (except, of course, when someone has forgotten to save them in Microsoft Office format!).
At one time, Microsoft's biggest rival for office software was Lotus. When the manufacturer ceased supporting Lotus, they placed their software within the public domain as 'open source'. It's now known as LibreOffice and is also well worth considering if you don't want OpenOffice. The Mac version is here:
http://download.cnet.com/LibreOffice/3000-18483_4-75337652.html