Currenly there is windows version only so your chance to play on osx is ether running it on wine or crossover game. You can count bootcamp too for running windows on mac.
I would love to see pso2 on mac too, natively. However I'm a die hard fan for PSO, This game might convince me to install bootcamp or get a new PC just to play it.
Right now I'm waiting to see ios or vista version. Also there's a little hope for it coming to ps3.
Edit:
You can also try latest VMware or Parallel Desktop but still recommend wine cuz it's free