Getting it for OSX
You need Tcl/Tk for Aqua, which is handily available pre-compiled. Get the version with "TclTkAquaBI " and the start of the name. That distribution swings both ways. No, um, the BI stands for "Batteries Included." It has some libraries that you need. Just because you have Tcl/Tk installed for running pd or whatever doesn't mean that you have the BI version, so download it to double-check. If you get an error on starting OSW that you don't have Iwidgets, then you need to go get the TclTkAquaBI. Don't go download source or anything for iWidgets unless you enjoy driving yourself insane. You must install TclTkAquaBI before you compile OSW.
you also need Jack Tools
You currently need to download and compile the source of OSW in order to run it. Get the source via CVS.
cvs -d:pserver:email@example.com:/cvsroot/osw login cvs -z3 -d:pserver:firstname.lastname@example.org:/cvsroot/osw co osw cd osw/src/macosx ./configure make sudo make install sudo make externals
Launch Jack Pilot, which came with Jack Tools. Click the "start jack" button. Then open the terminal and type "osw" at the prompt. Everything out to work.