Make sure you install XQuartz 2.7.7 or higher first.
You will need to download the latest version of osu! from here and place it inside the .app (osu!.app/drive_c/osu!/
)
Something like: curl -ks https://m1.ppy.sh/r/osu\!.exe\?`date +%s` > osu\!.app/drive_c/osu\!/osu\!.exe
Let osu! update and hopefully it should run :)