Office 2011 has known issues with 14.5.5 and earlier versions and 10.11 El Capitan, and 14.5.6 was released to address them and 14.5.7 is already available.
For Mac OS X 10.7 and later, Microsoft Office 2008 (the installers that come with it may not be compatible, but most files can be dragged and dropped) and 2011 are compatible from http://www.microsoft.com/mac and it has its own forums at:
Earlier versions of Microsoft Office will not work natively with Mac OS X 10.7 or later (Lion, Mountain Lion, Mavericks, or Yosemite),
that includes v.X and 2004 even though they might have worked with prior versions of Mac OS X. If you need to read newer Office documents,
see the alternatives below to upgrading to Office 2008 or 2011 to see if they might be cheaper or are able to fill the need. If migrating from PowerPC to Intel Mac, you should also be aware of this tip: https://discussions.apple.com/docs/DOC-2295
Note: Very few Office documents may require Office for Windows due to specific macros, or DirectX additions. If you get something that is unreadable, it either is due to that, or a version of Office newer than what you have.
Here are the most commonly referenced alternatives to Microsoft Office.
http://www.apple.com/support/mac-apps/ - lists Numbers, Pages, and Keynote as alternatives to Excel, Word, and Powerpoint.
http://www.openoffice.org/ - open source
http://www.libreoffice.org/- open source, and also includes support for Publisher.
http://www.neooffice.org/- open source
Word processing only:
Apple TextEdit (prebundled with Mac OS X) - yes it can open Word files, though formatting may be seriously hampered for all except the most simple documents.
Installing Windows on your Mac:
For Macs prior to July 20, 2011, Appleworks was also an alternative, though this tip had to be created to help people to migrate:
The https://discussions.apple.com/docs/DOC-6841 link mentioned on tip 2522
can also help those who have older versions of Microsoft Office for Mac OS X.
Microsoft Access has never been written for Mac OS, though Filemaker Pro has been around for a long time. People migrating from Windows
will be glad to know there is a tool to get Access to Filemaker Pro format before moving to Mac OS X: