I did both Oxford in 1998 and Guildhall in 1999.
At that time, it seemed OATS was only interrested in the students money. They couldn't care less about modulars. If you weren't CAP 509, or BA Cadets, forget it. Their notes were crap at the time.
Went after that to London Guildhall, where I met lecturers concerned about their ability to teach, rather than the RAF ex-navigators talking crap in order to be able to afford a pint I met in Oxford.
True, Guildhall doesn't shine as much as OATS.
But the quality of the lectures is so much better.
I did not fail one exam there out of 10 I sat.
Now, it was few years ago, but still, I recommend Guildhall, especially if you can buy OATS notes from Transair Pilot Shop down the road, or even borrow them.
Good Luck.