OP = Overall Position... the Queensland Tertiary Admissions Centre takes these into account when offering placement in university degree courses... the higher the OP, the more likely you are to get into the course you want. OP rankings are from 1 to 25 with 1 being the highest and 25 being the lowest.
Additionally, it is considered a generalised marker of intelligence, and as such OP ranking is also used in an exclusionary capacity, someone with an OP of, say... 23, is considered to be "too dumb" to study, for example, Information Technology or Medicine.
As a general rule, around 2% of students graduating each year recieve an OP1, around 4% receive an OP2, and so on, so forth, with the percentages increasing towards the middle of the range (from OP6 - OP19).
A guideline is that the higher the OP the better, for example, Information Technology (non specific studies) required an OP of approximately 13 (depending on the Uni)... Medicine (Bachelor of Medicine non specific) requires an OP of 5 at most unis, and a Bachelor of Vetinary Sciences (non specific) requires an OP2, at some unis even an OP1. I find it hilarious that Vets need to be smarter than Doctors

These OP rankings, percentages etc and all the info are what I remember from when I graduated from high school in 1999 and completed university, some of the stuff may have changed.