I've got osu! running on macOS High Sierra (10.13)
Sorry for the not-so-detailed instructions, but here's a basic guide...
If you have an osu!.app that doesn't work, don't delete it just yet.1. Download and install
PortingKit2. In PortingKit, create a new wrapper called osu (File --> New Custom Port) on the WS9WineStaging2.16 engine and Mac Driver Window System.
3. When prompted to select an installer, click cancel, yes, then cancel again. (You'll get a warning, that's fine)
3. Once the wrapper is created, open the wrapper, go to Advanced --> Tools --> Winetricks and search for "dotnet40", click the checkmark and hit "run" This installs .NET Framework 4.0. (This will take a while)
4. Download the windows osu! installer from
here.
5. Go back to the wrapper, and go to Install Software --> Choose Setup Executable and choose the osu!install.exe file.
6. Let osu! install into the default directory.
7. osu! will automatically open after it installs. Exit out of it.
8. For some reason, wineskin doesn't detect the launch exe, so you'll have to go to the wrapper --> Advanced --> Windows EXE and look for the osu!.exe file. (The directory should be /users/Wineskin/Local Settings/Application Data/osu!/osu!.exe)
9. Close out of the wrapper and open the osu.app itself.
10. Login and you should be able to play.
If you have an existing osu!.app and want to transfer your beatmaps and skins1. Right click --> Show package contents of the old osu.app and go to drive_c and find the osu! folder where your beatmaps and skins folder are.
2. Copy the beatmaps and skins folder and go in drive_c to the osu! folder in your new osu.app then paste (and replace).
3. Delete your old osu.app
Note that osu may be laggier than it was in Sierra. However, it seems that
all games seem to be running at lower fps since the update. Thanks Apple...
EDIT: Revised instructions