The OET writing subtest expects a certain level of professional but clinical approach from the test taker, and mistakes in these regards are quite common too.

Here is a list of common errors compiled just for you.

1) Multiple complex information in a paragraph.
When starting a paragraph always keep in mind to discuss only one aspect of the task in one paragraph. Never talk about multiple things in the same paragraph as this might compromise the effectiveness and the efficiency of communication of the letter.

Eg: Mr Sheffield is a golf player and lives with his family in a rented flat. His two children stay abroad. Mr Sheffield has been diabetic since 2008 and is on medications. He visits his GP twice a month for check-ups. 
(Here both social and media history is detailed. Keep your paragraphs thematic instead)

2) The start.
It is often a mistake that test takers make to start the letter with the patients medical history, even if the the need of the hour has nothing to do with his or her past. Understand the urgency of the situation and prioritise the information that needs to be presented first.

Eg: Regarding his medical history, Mr Steven has been hospitalised twice, one for a knee fracture and other for a flu. He has also had an appendectomy done in 2009. He reports to be allergic to peanuts and there were two such occasions when he had to be prescribed tablets for violent bouts.
(All these are not required/ relevant when writing to a physiotherapist)

3) The reference.
When referring to the patient, always pay adequate respect and have it in your mind to use the proper title. Referring to someone on a first name basis is considered rude but is acceptable in case of children. Under no circumstances should you refer to the person as "the patient".

Eg: The patient was admitted to hospital on 21 January due to syncope. He then underwent an ECG which revealed complete heart block following which the patient underwent a surgery.
(Use his name instead)

4) Avoid adjectives 
In OET, the facts are to be stated and not the adjectives. For example a "heavy drinker" might imply different amounts of alcohol consumption to different health care individuals. Thus always use clinical and factual language.

Eg: Mr Thomas is an alcoholic and a chain smoker. He has now complied to participate in the de-addiction programme conducted at your facility.
(Avoid being judgemental. Instead use quantity. Mr Thomas drinks 3 units of alcohol everyday and smokes 10 cigarettes a day.)

5) The Purpose
The healthcare individual you might write to will possibly be in some sort of hurry all day. So it is absolutely imperative that you make the purpose of the letter as clear as you can. The purpose serves an important role in the prioritising of the tasks.

Eg: This letter is to refer Mr Edwin Collard to your facility for continued care.
(A very ambiguous purpose!)

6) "Academic Style"
 Words like "Moreover" and "Furthermore" suit a style of writing that is inclined to the academic side. But remember, Professionalism is the need of the hour so make sure you replace these words with ones like "However" and "in addition"

Eg: He needs to be given pain killer twice a day. Furthermore, care has to be taken to ensure the patient follows a liquid diet and starts semi-solid food only two weeks later, after the first follow-up visit with the surgeon.
(Do not use 'furthermore')

7) Leaving out info in the final request.
Most letters will require the person at the other end to perform some sort of action. So before you finish the writing subtest, make sure you have checked off all the requirements of the letter from the case notes.

Eg: Kindly ensure that Mr Thomas follows a fibre-rich diet and that he is given continued care.
(This is such a vague ending)

If you avoid these mistakes, you can get a much better score in your OET letter writing.