To be pedantic, there are 8 different valid -O options you can give to gcc, though there are some that mean the same thing.

The original version of this answer stated there were 7 options. GCC has since added -Og to bring the total to 8

From the man page:

  • -O (Same as -O1)
  • -O0 (do no optimization, the default if no optimization level is specified)
  • -O1 (optimize minimally)
  • -O2 (optimize more)
  • -O3 (optimize even more)
  • -Ofast (optimize very aggressively to the point of breaking standard compliance)
  • -Og (Optimize debugging experience. -Og enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.)
  • -Os (Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size. -Os disables the following optimization flags: -falign-functions -falign-jumps -falign-loops -falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays -ftree-vect-loop-version)

There may also be platform specific optimizations, as @pauldoo notes, OS X has -Oz