I'm currently in the process of reading the book, "The Imp Of The Mind" by Dr. Lee Baer and he had some interesting things to say about the correlation between PTSD and OCD. Basically he says that the real difference between someone who has PTSD and someone who has OCD is that the person that has PTSD struggles with thoughts of events that have actually happened, whereas someone who has OCD thinks about imagined situations that might occur. People with OCD try to reassure themselves that a situation never occured, whereas someone with PTSD knows a situation occured and have trouble dealing with it.
You asked if someone with OCD could develop PTSD by simply thinking about non-trivial events constantly, and in my opinion I would have to say no. PTSD and OCD are two different issues. The reason I say no is because of what I stated above, which is there is usually a traumatic event associated with PTSD, but not with OCD.
Then again, sometimes there are underlying issues in people with OCD that can trigger their symptoms since symptoms of OCD can occur as early in age as 3, or in late in age as 65.
Hope this helps.