Can someone explain to me how the australian education system works with OP (OVERALL POSITION)?

Question

how is it calculated and what kind of subjects are involved are there optional and compulsory subjects

Answer 1

The OP calculations take into account a student's best 5 Authority subjects, that is, the 20 semester units in which they receive the highest scaled subject achievement indicators (SAIs).

In calculating OPs, all subjects are treated equally. There is no bias in favour of certain subjects (e.g. maths and science).

It is only used in Queensland, the other States have different versions.

Students must sit the Queensland Core Skills Test to be eligible for an OP rating.

Students are placed in one of 25 OP bands from 1 (highest) to 25 (lowest).

Those with the highest bands get the best University options.

Answer 2

OP is a Queensland thing.  As best I can work it out the schools just make it up.