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summary/ tl;dr at the bottom of the page

Inroduction

What is DID?
DID is the acronym for Disassociate Identity Disorder, previously known as Multiple Personality Disorder.
DID is defined as: a mental health disorder, which a person has two or more separate identities, in which these personalities have different appearances, history, backgrounds, behavioural patterns, likes and dislikes. DID is a form of dissociation, similar to (but seperate disorders, with some links) derealization/depersonalization disorder or dissociative amnesia. Other personalities can be internally percieved as different species or races, even ethnicity, as every alter has a different background and history, little, or none at all. (When a personality comes to be active, 'fronting', the other personalitie(s) (usually) experience a form of amnesia and cannot remember the occurances of another 'alters' front.
The causes of DID can vary, but are linked to the repeated psychological trauma of intense physical, sexual, or emotional abuse at a young age, around and under 6. These factors cause the brain to dissociate and starts creating other personalities to handle what has occurred, as DID is a coping mechanism / trauma response to protect the core person or body.

Required Diagnostic Symptoms are:
-displaying two or more distinct identities involentarily who have different patterns and ways of perceiving the world, thought processes, etc.
-reoccuring memory gaps in memory in everyday situations and events, personal/important information, unable to be regular forgetfulness.
-symptoms are not due to religiou or cultural practice, alcohol, drugs, another medical condition, or imagination/ fantastical play
-symptoms cause stress, struggle or problems in everyday events and relationships.
Otherwise Symptoms of DID include
- memory loss (especially when one alter (personality) is fronting while another is not)
-out of body experiences
-severe dissociation
-depression, anxiety, suicidal thoughts
-drug use and abuse
-lack of sense of self/ self identity
-emotional numbness & detachment
(some being side common symptoms, not required to fit diagnostic.)
Treatment includes talk therapy, and some medications such as antidepressant, , or individually fitted treatment, none known to rid of alters fully.

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What is OSDD-1?
OSDD-1 is the acronym meaning Otherwise Specified Dissociative Disorder, previously DDNOS (Dissociative Disorder Not Otherwise Specified).
OSDD-1 is defined as: individuals who have similar sympoms to those with DID but who do not fully meet full disgnostic criteria for DID. OSDD-1 describes: OSDD-1a, generally being individuals who have separated dissociative parts, but are not differentiated enough to be considered alters; and OSDD-1b describes individuals who do not have amnesia between these alters and share memories and relevant information (not strictly, as people with OSDD-1b may still experience situational, but not consistant, switch - based amnesia)
The causes of OSDD-1 are the same as DID, as OSDD-1 only functions differently from DID in missing diagnostic criteria only in areas of symptoms. with this being said- see the causes of DID to see the causes of OSDD-1.
The Diagnostic Symptoms of OSDD-1a are the same as the diagnostic criteria of DID, with the lack of strong enough differencitated parts to be fully formed alters, but still presenting different dissociative parts that can influence thoughts and actions in some cases, and the presence of amnesia.
The Diagnostic Symptoms of OSDD-1b are the same as the diagnostic criteria of DID, but with the lack of amnesia, where the other identities/alters can act as themselves, but all alters (for the most part) share memory and information.
Otherwise Symptoms of OSDD-1 are again the same as DID, with the lacking features of each subtype applied.
Treatment includes talk therapy, medications to treat additional treatable symptoms, or indivitually fitted treatment.

What is the difference between DID and OSDD?
Both of these disorders are trauma based, but DID and OSDD are typically cormorbid with C-PTSD. C-PTSD encompasses posttraumatic response on a neuropsychological level, and the relationships with others and the world due to it. DID and OSDD describe the interrupted development and different actions / symptoms that occur because of the trauma endured, showing the dissociative disconnect of a core survivor of a system that simple singlet (non system, non- osdd/did people) CPTSD surviors.
Both subtypes of OSDD are similar but not 'full blown' cases of DID due to their diagnostic symptoms, and all have the same causes and non-diagnostic or side symptoms (with lacking diagnostic traits of DID applied tothe subtypes of OSDD)

SUMMARY/ TL;DR

DID: trauma response/ involentary coping method, dissociative disorder. is the presence of different identities (alters) in a CPTSD/ young age occured trauma surviors headspace, able to take front of control of the body, usually to fulfill certain functions to aid the body.

OSDD-1: the presence of 'near DID', being a condition of almost the same as DID, only differenciated by lacking elements depending on the subtype

OSDD-1a: the condition of 'almost DID', only lacking the full formation of identities enough to be considered identities.

OSDD-1b: the condition of 'almost DID', only lacking the amnesia experienced beween fronts of alters.