The International Otology Outcome Group (IOOG) was founded in 2017 to encourage and facilitate international collaboration with regard to the surgical outcome of ear surgery. This report outlines the methodology and recommendations of the consensus-based categorization of tympanomastoid surgery produced by the IOOG. The IOOG Steering Committee used the acronym SAMEO-ATO to categorize tympanomastoid operations, representing the stage of surgery, approach, mastoid bone extirpation, external bony wall repair, obliteration of the mastoid cavity, access to the middle ear, tympanic membrane reconstruction, and ossicular reconstruction. A modified Delphi technique was used to obtain international consensus. The expert panels included the chairpersons from 21 otology societies. The approval rate of the SAMEO-ATO system from the otology societies was 95%. The SAMEO-ATO scheme was presented at the 31st Politizer Meeting for field testing. There were no objections or serious concerns raised. Some international otologists wished to see more surgical categories included to reflect the varieties of surgical techniques, but they accepted that it would make the whole system cumbersome. In addition to providing an international categorization of tympanomastoid surgery, the IOOG Steering Committee plans to introduce a common otology dataset that the international otology community could use to record their surgical outcome. The high level of international consensus on the IOOG categorization of tympanomastoid surgery supports this tool for surgeons to pool their surgical data into a large database for research and comparative audit.