Where in Qt Creator do I pass arguments to a compiler?
It isn't really that obvious.
Depending on your build system it's either in your qmake project file(.pro, standard for new projects) or in one of the CMake files (CMakeLists.txt, used by KDE and several other projects).
Using .pro:
QMAKE_CXXFLAGS += -O2
Using CMake:
set( CMAKE_CXX_FLAGS "-g -Wall")