-xO1

Preloads arguments from memory, cross-jumping (tail-merging), as well as the single pass of the default optimization. 

-xO2

Schedules both high- and low-level instructions and performs improved spill analysis, loop memory-reference elimination, register lifetime analysis, enhanced register allocation, and elimination of global common subexpressions. 

-xO3

Performs loop strength reduction, induction variable elimination, as well as the optimization done by level 2.

-xO4

Preforms automatic inlining of functions contained in the same file in addition to performing -xO3 optimizations. This automatic inlining usually improves execution speed, but sometimes makes it worse. In general, this level results in increased code size.

-xO5

Generates the highest level of optimization. Uses optimization algorithms that take more compilation time or that do not have as high a certainty of improving execution time. Some of these include generating local calling convention entry points for exported functions, further optimizing spill code and adding analysis to improve instruction scheduling.