OSSubprocess works on macOS Mojave with 64-bit Pharo.
Load it the usual way:
Metacello new baseline: 'OSSubprocess'; repository: 'github://pharo-contributions/OSSubprocess:master/repository'; load.
Run the tests. If Pharo was started like how applications are usually started on the Mac, by double-clicking the Pharo.app icon, Test Runner will report 40+ test errors.
Run Pharo the Unix way on the command line:
~/MyApps/Pharo.app/Contents/MacOS/Pharo mypharo7.image. Now all 114 tests
pass. All 7 stress tests pass too. Tested with Pharo 7.0 pre-release image.