It actually goes down to what course and what polytechnic you
are looking for. The entry requirements differs a little from poly
to poly and course to course.
The reason is because for some poly, the entry requirements says
you must have took the subject before in O levels
Eg:Click
here to look SP's Diploma in Business Admin entry
requirements
As you can see, it says other than EL, Maths, you need 3 other
subjects, and must have sat for the following papers stated there.
One of which is POA.
Eg2:
SP's Infocomm Security
You need EL, Maths, 2 other subject, and must hv took the
following papers, and one of them is D&T, but no POA. But it's
ok, because they said you can use combine science as one of the
subject also
Eg 3:
TP's ENgineering
You need EL, MAths, one of the subjects, which D&T is
included, but not POA.
So you can see it matters a little on what course you can take
in Poly. But don't give up on that option because it will not help
you to go into your course of interest in poly, because sometimes
the required subjects are already ur core o level subjects
As for what jobs you can have in the future does not really
depend on your o level subject, but the knowledge you learnt can
assist you.
By today's standard, taking POA in O level does not enable you to
be an accountant, nor taking D&T will enable you to be an
engineer. It only gives you the basic knowledge you need to study
further.
Good luck!