MOS stands for MIcrosoft Office Specialist and is a family of certifications from Microsoft Learning that deals with all the major office components — namely, Word, Excel, PowerPoint, Outlook, and Access plus SharePoint and Project. In fact, core Office components (Word and Excel) even offer Expert designations, and a Master certification is available for various major versions of Office (2010, 2007, and so forth) as well.
MOS was the designation for Office certs prior to the introduction of MS Office 2007, and is once again the designation for MS Office 2010 certifications. There was a brief hiatus during which these credentials bore the Microsoft Certified Application Specialist, or MCAS, designation but all those certs have now been folded back into the MOS Program. All MCAS holders qualify for similar MOS credentials for Office 2007, and can even order certificates to that effect from the company that runs the MOS Program for Microsoft — Certiport — for a modest fee through their login portal.
According to Anne Martinez’ latest Certification Watch (Vol 13 #8) the real reason for the name reversion is that “…the MCAS name never caught on.” My educated guess is that because the MOS program was around for such a long time, and Microsoft didn’t really go into a full-court-press rebranding effort, nobody bothered to change their nomenclature and the program handle stayed MOS in employers’ and cert-holders’ minds, even if that’s not what MS was calling the program during that brief interregnum.