The pattern that seems pretty consistent in the feedback from these boards is that if you are successful, you don't get any feedback. In fact, all your OASC paperwork makes up the opening inserts on your personal record, so when you retire you can ask for a copy of your Service Record and see them then! You can of course, ask for them under the Data Protection Act, at any point. They do however seem to offer feedback to near misses - which makes sense if you think about it. Who cares how you did once you have passed and are moving onto the next stage, but let's help returners actually improve their performance.
As to the scores, the highest score is given for a performance that equates to that which would be expected of a junior officer on graduation from IOT - ie bl**dy good. The lowest score (1 or 0) is as bad as it can get ie useless, no positive features, incompatible with military service etc.
Over the years, the scoring range has changed, in my day it was out of 5 with much use of plusses and minuses.
One thing that may surprise/confuse people is the mean scoring or the acceptance rate. Depending on the quality of candidates and the number of places available, the pass mark is likely to be at about 2+/3, at least for sifting. 3+ will probably get you an immediate offer. How come you might ask, when the RAF prides itself on setting such high standards? Well because they have a basically dismal view of civilians, and great confidence in their own training system, so they believe that they can convert a 'mere' 2+ civilian, into a top scoring junior officer by the end of IOT.