OSDD-1a, or other specified dissociative disorder type 1a, is one of five diagnostic labels clinically recognized to cause a system, along with DID/MPD, OSDD-1, OSDD-1b and USDD. It is a sub-type of Other Specified Dissociative Disorder type 1, wherein the members of a system are less differentiated than in classic DID. People who have this sort of experience but do not wish to label their system medically may describe themselves as being mid-contiuum or median.
Dissociative identity disorder is typically diagnosed if there is amnesia between the different identities, provided that the different identities have notably different senses of self. In OSDD-1a, the different members of a system typically share the same name, and may present as different ages or different modes of the same person, such as a "work" vs "home" self. There may be no consensus in an OSDD-1a system about whether a specific system member is the "main" member.