@Ragman,
There are different types of OCD
It's not must cleaning or checking or counting as a typical person would think. No there is Magical Thinking Intrusive Thoughts - is the fear is that even thinking about something bad will make it more likely to happen - sometimes also called ‘thought-action fusion’.
Sufferers are beset by intrusive bad thoughts. They try to dispel them by performing rituals - magic rituals, in effect - that are often bizarre and time-consuming and involve linking actions or events that could not possibly be related to each other. For example having the thought 'I may strangle someone' is regarded as being as reprehensible as actually strangling a person. Another example is believing that simply imagining a horrific car crash will increase the likelihood of such a crash taking place, or a person may feel that if they don’t count to ten ‘just right’ harm will come to a family member. Other examples of magical thinking, or thought-action fusion intrusive thoughts include:
A certain colour or number has good or bad luck associated with it.
Certain days have good or bad luck associated with them.
A loved one’s death can be predicted.
One’s thoughts can cause disasters to occur.
Stepping on cracks in the pavement can make bad things happen.
Whatever comes to mind can come true.
Breaking chain letters will actually bring bad luck.
Attending a funeral will bring death.
One can inadvertently cause harm to others with thoughts or carelessness.
Hearing the word ‘death’ will mean repeating the word ‘life’ to prevent death.
In each example listed above, the thoughts and events happening could not possibly ever be linked, but the person with OCD will believe that this possibility does exist, and as a result, this will cause them immense stress and anxiety. As a result, their silent internal compulsive behaviours will take hours, and often prevent them interacting with anyone else during this time.