Actually no one knows how to get into OCS, xcept the ppl at CPC,
perhaps they know best!
However some of the more common characteristic of those OCS cadets
are:
1) GOOD A-level Prelim results or poly grades
2) Chances are, if you are from NJ, HCJC, RJ... u will be
considered if u are not tat worse. But if you are from YJ, SRJC,
chances are, u are unlikely to be considered. Of coz YJ / SRJC top
students are exceptions. So it all boils down to your grades!
3) U have a good relationship with NCC / NPCC when u are in
secondary school. Best if u proceed to be cadet inspector or cadet
LT.
4) Be an SAF Scholar!!! 
These may be the general characteristic, but it is not exhaustive
and it may not be consistent with reality. However, it serves as a
good proxy for the Selection
Criteral for SAF Officers