In 2010, there were 3845 DO graduates and 261 previous grads, for a total of 4106 DOs looking for a residency. That same year, there were 1,801 AOA residency spots offered, 1210 spots were filled, leaving 591 AOA spots unfilled. By the way, there were 629 traditional rotating internships offered, but I didn't count those in the numbers I gave.
Just to give you an idea of the unfilled AOA spots...there were 339 unfilled Fam Medicine, 174 IM, 17 Peds, 12 Ob/gyn, and everything else was between 0-9 unfilled spots.
These numbers are pre-scramble, however.