OS X comes with zsh but not ksh. zsh can be made to emulate ksh much like bash emulates sh. If you look in /bin, you will find tcsh and csh are hard linked to the same (tcsh) file, likewise bash and sh. To get zsh to deal with ksh shell scripts, you can issue:
sudo ln /bin/zsh /bin/kshThis must be a hard link to function correctly. The zsh page suggests a soft link, which on other UNIX systems seems to work, but Darwin apparently requires a hard link for it to function properly. Like bash, zsh "senses" how you invoke it (ksh vs. zsh; sh vs. bash) and sets defaults for the shell session accordingly.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030608023948686