OCD is a disorder of the imagination. It occurs when your perception of reality is surpassed by your fearful imagination. OCD often involves intrusive thoughts that follow common themes (e.g. contamination, fear of going crazy, fear of making mistakes, fear of hurting someone else or yourself). To calm these intrusive thoughts and urges, people complete mental or physical rituals or compulsions.
For instance, someone who is worried they may leave the stove on and it will lead to a fire (intrusive thought) may engage in compulsions (e.g. saying a prayer; checking the stove many times; asking for reassurance from loved ones that the stove is off; take photos of the stove).
I know OCD can be extremely painful to live with, and am passionate about helping people with OCD. I work with exposure and response prevention and inference based CBT to help my clients.