The Collaborative International Dictionary of English v.0.48:

Io \I"o\, n.; pl. Ios. [L.; cf. Gr. "iw`.]
   An exclamation of joy or triumph; -- often interjectional.
   [1913 Webster]

The Free On-line Dictionary of Computing (17 December 2009):

IOS

   Internetworking Operating System


V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006):

IOS
       Internetworking Operating System (Cisco, OS)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006):

IOS
       InterOrganizational Systems