Thank you for the complements, Bad Wigins.
To try to answer the original question. First, nearly all diagnosis in psychiatry is at the syndromal level. In other words, a collection of symptoms and signs co-aggregate often enough to be recognised as an entity. Therefore, it is an approximation of a deeper biological truth, which we don't know that much about.
Secondly, the term psychosis implies the presence of characteristic symptoms such as delusions, hallucinations (usually auditory), thought disorder (a breakdown in language), and generally disorganised and/or bizarre behaviour.
Thirdly, psychosis can be a part of conditions such as schizophrenia, bipolar, drug addiction, medical illnesses, or, indeed, flare up in times of extreme stress.
Fourthly, with regard to delusions, definitions vary, but there are some useful guidelines. They do not have to be bizarre, but if something is completely 'crazy' and far out, then it's likely to be delusional, especially if the content is well out of keeping with their background, culture, and current situation. If it is stuff that could happen, even if unlikely, it gets trickier. A good clue is that their explanations for the odd stuff don't make any sense either. If you keep asking 'why', or 'what was that all about', or 'how come that happened' etc, you will often unearth strange and illogical/irrational answers that confirm their not in touch with reasonable, consensual reality.
Psychotic people often have a strange and intense demeanour, look and act weird, say things that leave you thinking 'and?', and their general function in various domains of life tends to be going downhill. Generally, they have little or no insight into the
In OCD, people may be fixated on certain obsessions, classically germs, stuff being turned off/locked, harm coming to people by their actions or inactions, order and symmetry. The compulsions arise to neutralise the obsessional content-handwashing, checking, avoiding certain people, situations or places, ordering things in a particular way, counting in a set pattern etc. However, they usually have insight into it being excessive, unusual and interfering with their life. In other words it is incongruent to them, and seen as an anomaly=insight.