The cut-off isn't very high (below 20) although no one can give you an exact figure. I know people who have managed to answer only half of the 30 questions and still ended up getting the interview.
However, what is important to know is that your score will be considered along with your CV and your quant grades from uni. If you have strong quant grades in your transcript, you might not need a very high score in the OW test. However, if quant grades weren't super high in your transcript, double down on the OW test and try to land closer to the 20s over 30 mark.
Best of luck!