Melbourne

There's a difference. OCD is Obsessive Compulsive Disorder. The most well known symptom is being compelled to do a certain thing over and over until it is "right", such as turn off a light switch multiple times until you get it "right", or washing your hands until they bleed because you can see "germs" still.

PTSD is when a person experiences and extremely traumatic event, such as war zone combat, victim of an assault, etc. the brain will rewire and enact pathways to "protect" you from the event/memory. This leads to the brain not storing the memory in long term memory, but always in short term, so flashbacks (essentially being teleported back to the traumatic event) occur frequently as the brain thinks the short term memory is a new event that is happening.

CPTSD is similar, but it's Chronic or Complex. This means a person has experienced trauma over a long period of time, or multiple times. I have this. This can be caused by being stuck in an extremely abusive relationship (be it romantic or familial), and the brain has a plethora of short term memories not stored correctly, so the triggers for a flashback are numerous and esoteric. The flashback won't just be one event, it could be multiple different events.

That's a simplified explanation.