To clarify first.
N level with full passes.
Normal Academic or Normal Technical ?
If Normal Technical, it is not advisable to do private "O" level
as the gap between NT and O level is very large.
If Normal Academic and you are willing to work hard ie to
spend a lot of time doing past year exam papers on top of attending
the private "O" level classes and working part time in the day, it
is possible. However, you need to realise that you will have
give up most or all of your social life and you need a lot of
physical and mental stamina too.
ITE is REALLY NOT it is the end. Do not be misled by
the image issue of ITE. ITE can really teach and you can really
learn a skill that can allow you to get a job easily.
As compared to ITE, a poor "O" level result with all F9, E8,
D7 will not get you anywhere or even a proper job.