It is as if the two parts of their brain are not connected. One part order the body to do a certain thing, lock the door for example. The other part of the brain does not registers this action. The person is not sure if he did it, or if he did it right and he repeats the action again. But before the repetition accrues, the stress level is building up and leading to anxiety.

Have I locked the door when I left the house? Have I turned off the stove or is the pot left boiling and will burns the house down? Thoughts like that in a constant loop can cause a lot of stress and interfere with normal life. People with OCD are aware of the fact that are repeating the same action but they are powerless to stop themselves.
The fears a person with OCD can range from fear of germs, fear of harming others, fear of being embarrassed in public, fear of thinking bad thoughts, among many others. They need order in their life and any deviation causes anxiety. Compulsion that an OCD person can engage in are excessive washing of hands, refusing to touch door knobs or shake hands, eating food in a certain way and in certain order and being “stuck” on a word, number or phrase without being able to move on.
Doctors believe that the cause of OCD is both physical and environmental. The serotonin levels in the brain are usually low. Low level of serotonin uptake in the brain is thought to be passes from parents to children. Recently researchers found that there might be a link between repeated ear infections in childhood (especially Streptococcus) may lead to OCD later in life.
The treatments for OCD are similar to the treatments for anxiety. Anti depressants that elevate the level of serotonin uptake, psychological therapy and acquired skills that teach the sufferer how to avoid repetition are also helpful. The combination of all three is the best way to help a person suffering from OCD lead a normal and productive life.

If the cause is a specific fear you can make a plan to try to avoid this fear. If it is economically driven, for example, make a plan or a budget to live by. Try to start saving, which will alleviate your fear somewhat.

If the anxiety is caused by some event in the past and it is playing like a loop in your head over and over again, you can will it to stop with techniques a health professional can teach you.
Accept the past for what it is – the past. You can not change it. Learn to live in the present, and as they say in Alcoholic Anonymous – “change the things you can and accept the things you can not change”.
Since there is such a strong body and mind connection you could do some physical things to help you cope with anxiety. The first and foremost thing is exercise. Exercise releases serotonin in our brain, and serotonin is responsible for our feeling of well being. If you manage to increase the level of serotonin with vigorous exercise, you will feel better.

Learn some relaxation techniques such as meditation, listening to music or engaging your mind by solving crossword puzzles or reading a book. Going to the movies is also a good distraction. Remember what you feel now you might not feel in an hour.
You must have enough rest to help you brain regain it’s balance. Lack of sleep has many diverse affects on the body and increasing the stress level is one of them.

Change the things you can change. Avoid caffeine, alcohol, chocolate and nicotine. All those tend to increase your heart rate.
Join social groups or volunteer for some charity. Being in the company of others sometime relieves the tension. Being alone tends to make things seem harder than they really are.

It is advised to keep a diary of how you feel everyday and what you did to change it. Looking back on your diary might show you some common reasons for your anxiety and might reveal some technics that work for you. You have to make sure your symptoms are not getting worse while you treat yourself.
When all those things don’t seem to help, ask for advice from a medical professional.