You need to be comfortable with numbers as they form a large part of what flying is about.
Good passes at 'O' level / GCSE / Standard Grade / National 5 in Maths, Physics, and some would say English is a sound basis for the ATPL exams.
What you do post 'O' level is not critical. Again demonstrate you have sufficient grey matter to succeed.
However anything concerning under water basket weaving does not count.
The Navigation paper is the acid test that your brain is wired up correctly for aviation. Probably fewest number of facts, but all about understanding.
GAVTHENAV will probably have a wee smile on his face should he read this in sunny Spain between classes.
SIN COS TAN is as difficult as it gets.
BA & Aer Lingus require a higher academic standard than stated above.