OCD: Oh, no, that's just TV OCD. I'm actually a disabling pattern of disturbing obsessions and repetitive behaviors that make it very hard to live a normal life. I'm a stone-cold freak.
Obsessive-compulsive disorder is frequently misrepresented in fiction. Most people are surprised to find that rituals are not the defining trait of OCD — in reality, it is an anxiety disorder that causes repeated, unwanted thoughts. The rituals (which may or may not have anything to do with order) are simply attempts to stop those thoughts. Fiction so exaggerates the "ordering rituals" part in OCD, depicting people who are tormented by books in their shelf that aren't in the exact right position, that people are downright dumbfounded to find out that the anxiety disorder behind the rituals actually exists.
Like other anxiety disorders, OCD exists on a spectrum and its presentation can vary greatly from person to person and from day to day (depending on how much stress the person is dealing with).
As well, a few other subtypes of OCD tend to get undue attention in fictional presentations, such as an obsession with contamination, which manifests in lengthy cleaning rituals, perhaps because this can be so clearly presented onscreen. People with contamination OCD are shown repeatedly scrubbing their hands with soap after touching any door handle.
There are other subtypes of OCD as well, such as struggling with unwanted thoughts (e.g . a priest tormented by erotic fantasies, or a babysitter struggling with visions of killing the children she cares for). Another subtype is obsession about unintentionally causing harm, such as worrying that the iron or stove element was left on.
Obsessive-compulsive personality disorder (OCPD), on the other hand, really is all about order. Whereas OCD is an anxiety disorder, with compulsions and behaviors that alleviate anxiety from unwanted or intrusive thoughts, OCPD is a personality disorder. The person is globally meticulous and nitpicky, desiring order in every aspect of their life. Where someone with OCD is aware their behavior is irrational (which causes more anxiety, which needs to be relieved, which leads to more irrational behavior...), someone with OCPD doesn't suffer distress from their behavior and views it as The Best Way Of Doing Things; distress with OCPD comes from someone screwing with their system, not from their behavior itself. OCPD used to be called anal retentiveness (aka being anal), based on Sigmund Freud's belief such behavior stemmed from potty training issues in childhood, but was later officially named OCPD after that explanation was discounted as baseless in the psychiatric community (actual cause is overactive in the prefrontal cortex), though it's still informally by laypeople. In real life, OCPD is one of the most commonly diagnosed disorders.
There's nothing to prevent someone from having both OCD and OCPD at the same time, however this is quite rare.
Experts liken OCD to a hungry beast, which hungers for compulsions to be acted on. While acting on compulsions may provide temporary relief, in practice, doing so strengthens their grip. Because of this, individuals with OCD are treated with cognitive behavioral therapy, with medication sometimes prescribed to aid the process. With treatment, obsessions become less prevalent.
Fiction can land all over the place on this one. Sometimes the person has one or two odd behaviors, sometimes the person is incredibly demanding and particular. Sometimes it's a crippling disorder that limits the character's ability to function, sometimes it's just a mild quirk. However, it's usually a Disability Superpower that turns the character into a super-detective or some such.
