Exactly what happened to me 4 years ago. I got D7 for Maths,
retook and got an A1.
I applied for deferment, but don't ask me under which category, I
forgot.
Finished the retake, went to Poly, the rest is history.
Its not easy to take Os as a private, even if its only one subject.
The stress is that if you fail again, you are gone. There is no
margin for error, you must push yourself to score well. Having said
that, it is entirely possible to score well.
Good luck 
