this quota was meant for important rail heads not covered by the PRS (passenger reservation system),
for example : in NE states, many hilly areas were not covered by the PRS and were hence given a separate quota (OS).. the reservation from those centres were done manually on a paper and forwarded to nearest PRS .
.
but with advent of technology, i...
more... doubt if there are any such important places still to be covered by the PRS... hence i consider OS quota (Specially on a Rjadhani a work of a super VIP)
(this is my 2 cent worth understand of OS quota, more can be looked in the faq section... views are welcomed)