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(nasm.info)Section 2.1.1

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2.1.1. The `-o' Option: Specifying the Output File Name
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   NASM will normally choose the name of your output file for you;
precisely how it does this is dependent on the object file format. For
Microsoft object file formats (`obj' and `win32'), it will remove the
`.asm' extension (or whatever extension you like to use - NASM doesn't
care) from your source file name and substitute `.obj'. For Unix object
file formats (`aout', `coff', `elf' and `as86') it will substitute
`.o'. For `rdf', it will use `.rdf', and for the `bin' format it will
simply remove the extension, so that `myfile.asm' produces the output
file `myfile'.

   If the output file already exists, NASM will overwrite it, unless it
has the same name as the input file, in which case it will give a
warning and use `nasm.out' as the output file name instead.

   For situations in which this behaviour is unacceptable, NASM
provides the `-o' command-line option, which allows you to specify your
desired output file name. You invoke `-o' by following it with the name
you wish for the output file, either with or without an intervening
space. For example:

     nasm -f bin program.asm -o program.com
     nasm -f bin driver.asm -odriver.sys

   Note that this is a small o, and is different from a capital O ,
which is used to specify the number of optimisation passes required. See
*Note Section 2.1.16::.