I don't know about other Universities but for NTU, you can find
out more here:
[1] A-Levels:
http://admissions.ntu.edu.sg/UndergraduateAdmissions/Pages/ALevel.aspx
[2] Polytechnic Diploma:
http://admissions.ntu.edu.sg/UndergraduateAdmissions/Pages/PolyDiploma.aspx
For Polytechnic, Diploma Result: 80% and O-Level: 20%.