OCD is defined as a mental preoccupation with an unresolved issue, where the affected person feels the urge to engage in some type of physical or mental activity in order to alleviate the perceived threats of that preoccupation.
The trouble is that the more engagement in the urge, the less satisfaction in having had satisfied that urge, therefore the more anxiety about its unresolved status, and more tendency to engage in the ritual. It is like a closed circuit, where the faster you run, the less you get to anywhere. Often, patients with OCD describe their symptoms in either physical (behavioral) or mental ways.
Examples of physical or behavioral OCD are: excessive hand washing, checking emails, checking locks, checking on the status of one’s health, etc.
Mental symptoms of OCD are often in the forms of thoughts and images: such as ruminating on whether what they’ve said in a conversation has been offensive to others, or whether they have gone over a check list enough times in their mind, or going over the image of an accident over and over in the mind.