A person having such a disorder, always feels that something is wrong, I mean, the thoughts occupying their mind, are becoming more and more disturbing, not giving to live a normal life. A person starts to realize he or she almost doesn't have time for any other thoughts except these. I've seen also a great article at SupremeEssays describing the most various symptoms, the causes and possible treatment of OCD, you may look it up for more info as well.
(please remember that in order to be viewed as possible symptoms of an OCD, actions and thoughts listed below must be intrusive, causing distress, unwanted, impossible to maintain control over, and repetitive. Rather often, OCD victims can critically evaluate their thoughts and actions, assuming that they are absurd or unnatural, but can do nothing to stop them).
- Constantly checking electric/gas appliances and water taps in fear that they might be not turned off and thus could cause fire/explosion, flood, and so on.
- Counting people, objects, items, and so on.
- Repeating words and phrases over and over again.
- Believing that one’s thoughts may harm other people, and thus by all means avoiding “bad” thoughts.
- Fear of contamination and actions aimed at “maintaining cleanliness” (washing hands is the most typical).
- Obsessive striving for symmetry, order, and perfection.
- Thoughts regarding one’s sexuality (a wide range of fears including fear to become a paedophile; fear of having sexual relationship with members of one’s family; fear of becoming gay, and so on).
- Thoughts about harming other people (most often family members, children, women).
- Fear of having a mental disorder.
A person with OCD must remember that having such thoughts does not mean that this person can/will commit any of those actions they are afraid of. Usually, what OCD victims fear is what they would actually never be able to do.