Firstly, the bad news. I was asked how many questions would need to be correctly answered to get an 'A'. There is no simple OET marking template meaning that there is no simple answer to this. The easiest way to explain how your mark is obtained is to direct you to two articles.
1. This is the first (long) article which explains the marking technique: Decision Making in Marking Open-Ended Listening Test Items: The
Case of the OET
2. The second one is a file from OET explaining the test. Bear in mind that the article is from 2009, however, it is still valid. Important points are below: