Took me a bit to find one, so I’m noting it here in case I ever forget (or anyone else needs to find something like this). GNOME terminal works great (ctrl-click on the s/key challenge and it does the rest), but for OS X there wasn’t something so obvious and the only one I could find was SkeyCalc which is ok, but I don’t think it’s been compiled in years (last version it notes is 10.0), and it crashed a bunch on me.
So looking for (and this is for the sake of search engines and other searchers!) a One Time Password (OTP) S/Key solution for OS X, I came across Ski which has been updated recently and does pretty much the same thing SKeyCalc does. And it doesn’t crash.