How has borrowing from so many different domains of psychology affected the field of occupational psychology?
Maybe I should start by explaining how Psychology as a whole has been affected by it's input from sub-categories in Psychology, and then move on to it's current occupations.
There are many areas of Psychology, ranging through cognitive [studying brain processes], psychoanalysis [the more popular interpretation of occupational psychology - the shrinks], social psychology [psychology as a whole when focusing on human behaviour in groups] to neuropsychology [focusing on the brain as a physical body], etc.
In my perspective, the most major contribution to psychology's occupational industries is Poppers' scientific method - stating that unless there could be an observable, objective and measurable way to infer meaning and implications from behaviour, then it should not be judged to be scientific. This is the view of the behaviourist, whom generally speaking will have a distaste for any theories which can't be proven beyond doubt.
This has affected the industry in a large number of ways - but mainly, to change the path of psychology as more of a thinkers' philosophical endeavour, to a pathway for science. Without Popper's behaviourist manifesto, psychology to this day could be just as subjective as philosophy, with no - one being able to agree on very much at all.
Take Freudien theory for instance, although Freud's works are now very mainstream in the psychoanalysis proffesion, they were not based entirely on scientific observations but Freud's personal accounts of the human condition, and therefore was subsequently rejected by many major behaviourists of the day, and thousands of psychologists in the preceeding century. However, the reason that many of Freud's offerings [such as notions of transference, some acknowledgements of certain forms of repression, etc] have been accepted into a medical proffession today is through scientific studies carried out later on in the century - some in attempts to disprove Freudian theory, and some to back him up.
Another major domain that has been affective to Psychology as a whole is the cognitive approach. There is no sub-field of psychology that neglects the importance of understanding brain processes. From understanding how and at which normative rate children develop languages, to getting a better understanding about how human attraction functions, cognitive psychology is psychology's bread and butter to many people. The underlying, automatic processes that occur on another level from, (but still affective to) conscious thought control and restrict the way we as humans behave. At the bottom of virtually all psychological experimental studies, there are cognitive implications, which are regularly of great importance to the purpose of, and meaning behind the study in the first place.
So you get an idea of how important it is in this field to borrow from theorists of other psychological disciplines, in order to achieve advancement. If you want to see a picture hidden within an incomplete jigsaw, sometimes you have to switch corners for a while. Occupational psychology nowadays IS a science. Full stop. Although theories are welcomed, you will be expected to back novel concepts up with at least some evidence, whether it be experimental findings, or a theoretical model affording enough potential to warrant closer inspection.
However, if one were to be quite articulate, mathematics is the most important part of today's occupational psychology, and frankly, I cannot see that changing. The only way to improve on what we already know is to focus on some given hypothesis, obtain some data by observation of some kind, and analyse that data using statistics. Without statistics, psychology could not advance.
In conclusion, the affects of borrowing and lending of information between separate fields within psychology (and indeed, science as a whole), is growth of the field itself. It is the quickest way for the discipline to gain notions of truth, as well as credibility. It allows fields such as neuroscience to help psychoanalysists and remedial workers appreciate and appropriatly tend to those suffering with certain brain disorders. It gives psychoanalysis the opportunity to extend it's reaches into the cognitive circuit, in order for cognitive based therapies such as music / relaxation therapy to be brought into hospitals up and down the United Kingdom. This sharing of knowledge gives us the tools to predict, understand and even control [sometimes for the worse sadly; advertising, manipulation, etc] behaviour and get closer to the truth behind the biggest questions in psychology of any given present time.
Thanks for listening. I hope that you have found this somehow helpful.