The key difference between OCD and post-traumatic stress disorder (PTSD) is that the intrusive thoughts of people with PTSD are of content relating to traumatic events that actually happened to them, whereas people with OCD have thoughts of imagined catastrophes. PTSD patients with intrusive thoughts have to sort out violent, sexual, or blasphemous thoughts from memories of traumatic experiences.
The primary difference between OCD and PTSD is that intrusive thoughts in PTSD patients pertain to actual traumatic experiences they've endured, whereas those in OCD are about imaginary catastrophes. Patients with PTSD who have intrusive thoughts must filter out violent, sexual, or blasphemous thoughts from their memories of trauma.