ATO (Accredited Training Organization) is the abbreviation given to official accredited training institutions for the ITIL framework.
Accredited Training Organizations (ATOs) are permitted to display official logos and are listed on the web site
managed by the APM Group (APMG) - the official ITIL accreditor. Any organization that attempts to sell ITIL education as being accredited that is not listed on the AMPG official web site is doing so in breach of the ITIL copyright and is liable to legal action.
ATOs have to go through a rigorous application process AND have to have their trainers assess and accredited before they are eligible to deliver any official ITIL education services. Ask to see your trainers letter of assessment and approval from one of the official Examination Institutes (EIs).
The APM Group is the official ITIL accreditor; following it's appointment by the Office of Government Commerce (OGC) in 2006.