The $O directive controls code optimization. In the {$O+} state, the compiler performs a number of code optimizations, such as placing variables in CPU registers, eliminating common subexpressions, and generating induction variables. In the {$O-} state, all such optimizations are disabled. 

Other than for certain debugging situations, you should never have a need to turn optimizations off. All optimizations performed by the Delphi compiler are guaranteed not to alter the meaning of a program. In other words, the compiler performs no "unsafe" optimizations that require special awareness by the programmer.