clang_optimization

Dear
I am a student form Hrbin China. I hava some questions for you,please help me!
as we know:
clang -emit-llvm test.c -o test.ll -S **-**O3
opt test.ll -o test.bc -O3

Do these two O3 have some differences? I have checked at the llvm.org, but i could not find the answer…
I am looking forward for your earliest reply!