Hi,
OSDD and DID are dissociative disorders formed by repeated trauma in the developing stages of childhood. Children in early stages of development have multiple personalities that are not completely fused. While people without this disorder (Singlets) eventually fuse, people with these disorders do not. As a trauma response these personalities (alters) develop into beings of their own, with separate thoughts, feelings, and identities. They all share a body, though often are not all conscious at the same time. When one personality is not conscious, they experience amnesia, either not remembering at all what happened or lacking emotional memory.
What differentiates roleplaying and systems is that one is a literal mental illness. Roleplaying is one person choosing to act as someone else. They can choose to stop pretending, and control what they do. Alters in a system cannot control the other alters in all cases except OSDD-1B, and cannot stop "acting" like someone else, because it isn't acting. Additionally, people roleplaying don't experience amnesia.
Regards,
Adam