I've met several people who have PTSD and later find out they also have an OCD diagnosis, so yes, comorbid disorders.
PTSD has an obsessions component, and both are listed as anxiety disorders; however, I think I read that the DSM is going to put PTSD under a new category called Stress Disorders with trauma-induced, non-biological orgin disorders, such as DDNOS, DID, etc.
OCD can arise from a strep infection, trauma, and genetics. It seems anxiety disorders cluster in my paternal family tree, and Bipolar on my maternal side. I seem to fall under the anxiety side, but mother, sister, and grandparent are narcissist/bipolar or PTSD/Bipolar.
On the father's side, there is lots of PTSD/OCD in the family. Lots of military, hunting, and domestic violence as well. I am looking at patterns only, not causation or links between elements. Just trying to map out the genetic map of disorders that I inherited.
Also, I seem to be attracted to OCD people, and my spouse may have OCD. We're thinking maybe he needs to be diagnosed first before we assume anything. Also, I have some OCD traits, and am continually told they are PTSD only, but I tend to hide them from people, including therapists, so I don't know.