Well, but sometimes obsession with a loved one can be OCD. I was in an OCD support group years ago and there was a woman there, poor thing was tortured- pure intrusive thoughts of killing her parents, whom she loved very much and was not even angry at. The woman was diagnosed with OCD.

OCD, PTSD, anxiety, paranoia.....any of these can manifest with intrusive thoughts. That's why a pdoc is the best person to actually give a diagnosis.