“As an undergraduate student in psychology, I was taught that multiple personalities were a very rare and bizarre disorder. That is all that I was taught on ... It soon became apparent that what I had been taught was simply not true. Not only was I meeting people with multiplicity; these individuals entering my life were normal human beings with much to offer. They were simply people who had endured more than their share of pain in this life and were struggling to make sense of it.”
― Deborah Bray Haddock, The Dissociative Identity Disorder Sourcebook
Other specified dissociative disorder (OSDD) is a dissociative disorder that serves as a catch-all category for symptom clusters that don't fit neatly within another dissociative disorder diagnosis. This diagnosis used to be known as dissociative disorder not otherwise specified (DDNOS). Both OSDD and DDNOS contain(ed) examples of specific possible symptom clusters within the diagnosis. These examples were and often still are used as subtypes of the disorder.
For clarity, while OSDD-1 is the term that will be used throughout this webpage, what's being referred to here is what would have been known as DDNOS-1 before the DSM-5. This page refers to both OSDD-1a and OSDD-1b, the subtypes of OSDD that are relevant to dissociative identity disorder (DID). OSDD-1 is the specific subtype of OSDD that applies to individuals who appear similar to those with DID but who do not meet the full diagnostic criteria for DID. For those with OSDD-1a, this is due to a lack of two or more sufficiently differentiated alters, and for those with OSDD-1b, this is due to a lack of amnesia between alters. Some individuals with OSDD-1 lack both amnesia and highly distinct parts, and other individuals with OSDD-1 have highly distinct parts but rarely or never switch between them. Both of these later presentations are usually associated with OSDD-1b if such a specifier is applied.
Before the diagnosis of DID was expanded with the DSM-5 to allow for switching to be reported by the dissociative individual instead of witnessed by a diagnostician and to allow for a broader definition of amnesia between alters, many individuals who would now fit the criteria for DID were instead diagnosed with DDNOS-1. It was found that the majority of DDNOS cases were actually misdiagnosed DID cases (Spiegel et al, 2011, p.838). Even under the DSM-5, individuals with DID who experience mostly passive influence instead of switching or who are amnesiac regarding their amnesia might be incorrectly diagnosed with OSDD-1.
The most important difference between individuals with DID and OSDD-1 is the way in which they experience their alters. While alters for individuals with DID can be highly distinct and individual, dissociated parts for those with OSDD-1a might present as the same individual at different ages, as the same individual in different modes, or as the same individual reacting with different learned responses to trauma. Even individuals with OSDD-1b might experience their alters more as different versions of themselves, though they're more likely to experience a noticeable change in skills, memory, temperament, or overall personality. It is important to note that even the least differentiated OSDD-1b parts are still more differentiated, separate, and autonomous than the most developed parts that can be present in borderline personality disorder, posttraumatic stress disorder, or complex posttraumatic stress disorder, none of which involve dissociated parts that have a unique sense of self or self history. Even OSDD-1a parts are often more differentiated than the pure dissociated trauma materials, emotions, or attachment needs present in these latter disorders. As well, some individuals with OSDD-1b do have highly distinct alters such as those often found in DID. For these individuals, the main defining factor is their lack of inter-identity amnesia.
While individuals with OSDD-1a are rarely or never aware of their alters' activities, individuals with OSDD-1b do not black out or lose time. They may or may not have dissociative amnesia for aspects of their trauma history, but information flow between alters in an OSDD-1b system is much more consistent. While someone with DID might not realize that or when they lose time, someone with OSDD-1b is usually fully aware of their alters' activities and is very unlikely to find any evidence of unremembered activities. If they ever were to have a dissociative trance or fugue episode, it would be unassociated with their alters. They tend to struggle less with their memory on a day to day basis.
Because alters for individuals with DID are more dissociated from each other, individuals with DID are able to have fewer alters co-conscious at once. While someone with OSDD-1b might be able to have all of their parts present and aware of each other and the outside world at once, someone with DID might be able to reach only a fraction of their system at any given moment and may be unaware of or unable to communicate with a large number of their alters. In general, individuals with DID are more dissociative and score higher on measures of depersonalization, derealization, dissociative amnesia, identity confusion, identity alteration, and somatization when assessed with the SCID-D or MID. In regards to the theory of structural dissociation, it's assumed that individuals with DID are far more likely to have multiple apparently normal parts (ANP) and multiple emotional parts (EP) while individuals with OSDD-1 usually have only one ANP but multiple EP. The inter-identity amnesia and increased complexity of alters in DID systems are generally attributed to the presence of multiple ANP.
Counterintuitively, the higher levels of dissociation and the presence of multiple ANP associated with DID may lead many individuals with DID to at first present as higher functioning than many individuals with OSDD-1. This is because the ANP are dedicated to maintaining a "normal" life and are highly phobic of and avoidant towards the trauma-containing EP. The ANP's constant and severe dissociation helps them to achieve this. Individuals with OSDD-1b have a harder time escaping from knowledge of their trauma and the internal reactions which this incites. Of course, their ability to work with their parts gives them an advantage in processing their trauma and healing. On the other hand, those with OSDD-1b may be plagued by doubt and shame regarding their "me-but-not-me" parts and the associated actions for which they have full knowledge but over which they feel they have no control.
Before the DSM-5 was released, Rob Spring wrote an article for Positive Outcomes for Dissociative Survivors (PODS) exploring the differences between the diagnoses of DID and DDNOS. Though the DID diagnosis is now more broad than it has been in the past and so now captures some individuals who before would have been wrongly diagnosed with DDNOS, many points made within the article are still relevant.
The article can be found here: http://www.pods-online.org.uk/didorddnos.html