Knock can take different forms, some people can suffer from a single theme and others can have several at the same time or even migrate to another theme over time. 

Here are the most common ones that can be obsessed with:

Aggressive thematic obsessions, typical of OCD which is also called "impulse phobia"

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• Fear of harming oneself or harming others (in the present or the future);

• Fear of having done harm (in the past);

• The fear of going crazy, losing control and doing things you would regret;

• Aggressive and violent thoughts, images or words (which may include insults and / or obscenities);

• Images of aggressive acts (hitting a passerby while crossing them on the street, for example);

• Imagine insulting or offending others, stealing or betraying them on impulse (or not), or any other socially reprehensible act;

• Imagine rejecting, divorcing or cheating on the person you love and with whom you share your life;

• Deliberately hoping that other people will get sick, have an accident or die;

• Etc.

Sexual-themed obsessions

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• Evil and forbidden (illegal) thoughts, images or impulses;

• The fear of having had (without remembering it) or having on the spur of the moment criminal or deviant sexual behavior such as pedophilia, bestiality or incest;

• Fear of being homosexual (gay or lesbian) or of acting homosexual and / or having obsessive doubts about one's sexual identity (such as fearing that one's homosexuality is “repressed”);

• Having sexual thoughts about people around you (when you have no attraction for them);

• Etc.

Obsessions with a theme of danger, loss, injury or embarrassment

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• The obsessive fear of having an illness, an accident or dying, or that another person has an illness, an accident or dies;

• Fear of losing control and harming oneself or others (as in the category of “aggressive obsessions”);

• Fear of harming yourself or others because of your neglect or because of your thoughts;

• The obsessive fear of suffering because of others or of being rejected;

• Fear of suffering damage to his property (his house for example);

• Fearing that you will never be able to be happy or get what you want;

• Doubting about having harmed or harmed others in the past;

• Fear that others will harm us, betray us, deliberately lie to us or have already insulted, betrayed, lied or taken advantage of others;

• The fear of being rejected by the loved one;

• Fear that objects in our environment have been moved or changed in an inexplicable way;

• Suffering damage to what you own or being stolen;

• Being afraid of forgetting important information (memories, dates, etc.);

• Being trapped in an unsatisfying life or romantic relationship;

• Fear of acting inappropriately in public;

• Being afraid that his / her child (ren) will not be his (s);

• Being viewed critically by others (being criticized, judged);

• Etc.

Religious-themed obsessions

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• Fear of deliberately sinning or blaspheming (especially “on the spur of the moment”);

• Fear of having acted immorally (especially of having committed these acts without remembering them);

• Doubting their faith or religious beliefs;

• Having obsessive doubts about living your faith properly;

• Being afraid of being contaminated by evil or by the demon;

• Having unacceptable thoughts about religious figures, one's religion or God;

• Being afraid of being possessed;

• Etc.

Perfection themed obsessions

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• Ask yourself if you have said, done or thought certain things perfectly;

• Ask yourself if others fully understand what you are saying;

• Wanting to say or do certain things (or all) perfectly;

• Look for a perfect appearance (clothes, hairstyle, etc.) and / or focus disproportionately on what has been identified as flaws or bodily imperfections (typical obsessions of dysmorphophobia);

• Ask yourself if you have told the perfect truth;

• Look for a perfect order in things (in storage, for example);

• Keep your house perfectly clean at all times;

• Order things symmetrically;

• Etc.

Relationship / love-themed obsessions

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• The obsessive, excessive, sickly jealousy which is accompanied, for example, by thoughts, scenarios and images in which our partner is unfaithful to us;

• Having an obsessive fear that our partner is not interested or no longer interested in us;

• Obsessively doubting our love for our partner (eg: saying to ourselves "I don't love him anymore") and looking for evidence that we love him (when we consider that we really love him), an obsession typical of the couple's OCD;

• To have the image or the thought of a person other than our partner and to have the obsession to love this person while we really love our partner;

• Etc.

Note that the obsessions with aggressive / violent, sexual, religious, love and perfection themes like those we have just seen in the above lists are very present in what we call the "OCD of thoughts" (or Mental OCD).

Contamination-themed obsessions (which is also sometimes called the “cleanliness TOC”):

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1. Fear of being contaminated as a result of contact with:

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• Dirt (earth, mud, garbage, etc.);

• Secretions (saliva, sweat, urine, blood, etc.);

• Microbes (bacteria, viruses, etc.);

• Pollutants or contaminants (lead, radiation, toxic products, etc.);

• Household products (solvent, insecticide, etc.);

• Poisonous gases (pollution, car exhaust pipe, etc.);

• Greasy and / or sticky substances;

• Medicines (or the effect of their ingestion in the past);

• Food that has been “spoiled” by others (poisoned, soiled, etc.);

• Broken glass;

• Poisonous plants;

• Living or dead animals or insects;

• People;

• Etc.

2. Fear of contamination relative to fear of disease:

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• By contracting a known or unknown disease;

• Spread a disease and infect others;

• Getting sick while visiting a hospital, doctor's office, etc.

• Being contaminated by thoughts about being hurt or getting sick (yourself or loved ones);

• Being contaminated by certain words or by the name of certain diseases;

• Being contaminated by seeing a sick, injured person, etc.

• Being contaminated by the memory of a person who has died;

• Etc.

Obsessions with a neutral theme

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• Words, sounds or music that don't mean anything in particular;

• Obsessive images that mean nothing in particular;

• To count incessantly and without reason;

• Repeatedly asking yourself questions that are unimportant or unanswered;

• Pay constant attention to your environment (sounds, objects, colors, etc.);

• Pay constant attention to your thoughts;

• Pay constant attention to the normal (breathing, heartbeat, etc.) or abnormal (pain, tinnitus, etc.) functioning of your body;

• Etc.

Superstitious or magical themed obsessions

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• Being afraid that bad luck will befall you or your loved ones;

• Lucky or unlucky colors and numbers;

• Lucky or bad luck objects;

• The possibility that thinking or hearing bad news could cause it to fall on you or your loved ones;

• Believe that certain words, names or images carry bad luck;

• Believe that certain actions or behaviors bring bad luck;

• Places, people or objects associated with unlucky events and believe that their contact can cause bad luck;

• The need to perform certain activities a specific number of times;

• Etc.

The obsessions that focus on health and the body present in particular in dysmorphophobia and hypochondria

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• Obsessing over the possibility of parts of your body becoming ugly in some way;

• Being afraid that your body will produce bad smells (breath, genitals, etc.);

• Fear scars;

• Keep asking yourself if certain parts of your body are working (in general) or are working properly;

• Having parts of the body that are too large, too small or asymmetrical;

• Being too fat or too thin, going bald, etc.

• Being afraid of accidentally choking or vomiting;

• Being afraid that parts of your body are aging prematurely;

• Being afraid of suffering from a serious and undiagnosed illness;

• Etc.

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Source: https://www.nicolassarrasin.com/liste-obsessions-trouble-obsessionnel-compulsif-toc