I have friends who got Cs for their O levels English and still
got into local university
NTU takes your O level results into account when you apply for
their course, your o level results makes up 20% of your
'admission', you can read about it here:
http://admissions.ntu.edu.sg/UndergraduateAdmissions/Pages/PolyDiploma.aspx
What matters most now is that you maintain a high GPA for the rest
of your years in poly, as 80% of your GPA makes up the university
computation score
Dont worry too much, how bad is your O levels?
Recently I still got accepted by NTU, i enrolled with an not very
good GPA of 3.61, and i didn't score very well for O levels, i got
about 21 points for L1R5.
Work hard and you can still go into uni.