AFAIK, and I don't "K" very much, the only assembly-language code included in OpenBSD is in very specific places, such as boot blocks. See the assembly language (.S) files in src/sys/arch/i386/i386/ for example.
The assembler included with the comp*.tgz file set is /usr/bin/as. It's man page is
AS(1), and it is part of the GNU binutils.
The Netwide Assembler can be found in ports/packages: devel/nasm. Also in devel/ are yasm and fasm.
It matters not what assembler you use. It matters what you assemble.