Minimum GPA Requirements to enter local university varies from
course to course. You have to look at the latest IGP table
available from the university's website. From there you can find
the cut off grade.
NTU:
http://www3.ntu.edu.sg/oad2/website_files/IGP/NTU_IGP.pdf
As for O levels, it is hard to say. I have friends who didn't
score too well for O levels, went to ITE, poly, then finally NTU.
However they went for interview in NTU as their GPA isn't very
high.
What course do you have in mind?
I was accepted into NTU B.Eng EEE program in 2012, with GPA of 3.6,
O levels 18. Lowest cut off for that year was 3.4x if im right.