Obsessions are recurrent, unwanted thoughts, images, or impulses that the person regards as inappropriate, forbidden, or horrific. The obsessions trigger anxiety and other distressful emotions because the content of the thoughts are quite contrary to those that the person usually has. The person realizes that the obsessions are a result of their own mind, yet they are uncontrollable. Common obsessions are:

• Fears of contamination from dirt or germs
• Worry about this being careless or irresponsible
• Intolerance for lack of symmetry or order
• Fear of committing violent, sexual, or sacrilegious acts
• Fear of throwing away useless items

However, the themes of obsessions can be unique to the individual. Their main features are fears of future events that are highly unlikely or impossible.

Compulsions are excessive and inappropriate actions or thoughts the person performs to relieve the anxiety from obsessions, or to prevent something "bad" from happening. Common compulsions include:

• Excessive washing and cleaning
• Excessive checking for safety (i.e. doors locked, appliances unplugged)
• Rearranging objects
• Mental activities such as counting, praying, and replacing bad thoughts and images with good ones
• Hoarding useless objects, papers, etc. for fear of discarding something important or valuable

In addition to obsessions and compulsions, the following must be present for the diagnosis of OCD:

• The person recognizes that the obsessions and compulsions are excessive or unreasonable
• The obsessions and compulsions cause significant distress, are time-consuming (take more than one hour a day), or significantly interfere with the person's normal routine, occupational (or academic) functioning, or usual social activities or relationships.

Depression and other anxiety disorders, notably Social Anxiety Disorder and Generalized Anxiety Disorder, frequently occur with OCD. Other disorders that may fall within the obsessive-compulsive disorder spectrum include compulsive hair pulling (Trichotillomania), Body Dysmorphic Disorder (preoccupation with one's appearance) and gambling, sexual behavior, and compulsive shopping/shoplifting.