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(nasm.info)Section 2.1.16

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2.1.16. The `-On' Option: Specifying Multipass Optimization.
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   NASM defaults to being a two pass assembler. This means that if you
have a complex source file which needs more than 2 passes to assemble
optimally, you have to enable extra passes.

   Using the `-O' option, you can tell NASM to carry out multiple
passes.  The syntax is:

   * `-O0' strict two-pass assembly, JMP and Jcc are handled more like
     v0.98, except that backward JMPs are short, if possible. Immediate
     operands take their long forms if a short form is not specified.

   * `-O1' strict two-pass assembly, but forward branches are assembled
     with code guaranteed to reach; may produce larger code than -O0,
     but will produce successful assembly more often if branch offset
     sizes are not specified. Additionally, immediate operands which
     will fit in a signed byte are optimised, unless the long form is
     specified.

   * `-On' multi-pass optimization, minimize branch offsets; also will
     minimize signed immediate bytes, overriding size specification
     unless the `strict' keyword has been used (see *Note Section
     3.7::). The number specifies the maximum number of passes. The
     more passes, the better the code, but the slower is the assembly.

   Note that this is a capital O, and is different from a small o,
which is used to specify the output format. See *Note Section 2.1.1::.