The Oxford University primary Single Sign On (SSO) username gives the owner access to a range of University and College on-line resources and beyond. It is never re-issued to anyone else once you have left.
This page documents the current process of issuing the SSO username and is primarily aimed at Card Administrators and Department and College IT Support Staff and gives fuller description than http://www.oucs.ox.ac.uk/registration/univcard.xml
An SSO username is only issued to people who have been confirmed by the Department or College as having a status which qualifies them for one. The status is confirmed by the issuing of a University Card by the Administrator for the Unit.
For what each status is entitled to, see Computing Entitlements. If you are entitled, you may be able to request additional 'project' usernames, to enable you to separate different roles eg Administrator, or for a publication or research project. These would be in addition to your primary SSO account, and could be transferred if you leave.