There are so many different types of obsessions and compulsions when it comes to OCD. Perhaps among the less talked about are sensorimotor, or body-focused, obsessions which involve a heightened awareness and focus on involuntary bodily activities and processes.
Common examples include hyperawareness of swallowing, breathing, or blinking. Additionally, overattention to bladder and digestive processes -- indeed, any unhealthy focus on a specific body part or organ -- might also fall into the category of sensorimotor obsessions.
I think these types of obsessions seem particularly brutal because they involve necessary, ongoing bodily processes. There truly is no escape, and this fact often plays into the sufferer's obsessions.