FYI, I am using the version pinned with FSL 6.0.4 (i.e. v4.0.15-dirty Fri Jul 17 12:23:03 2020), and some of the output text files generated by oxford_asl which contain mean perfusion/arrival values seem to have typos in their filenames.
E.g.,
arrival_wm_wm_mean.txt (should be arrival_wm_mean.txt)
perfusion_wm_wm_mean.txt (should be perfusion_wm_mean.txt)
perfusion_wm_calib_wm_mean.txt (should be perfusion_calib_wm_mean.txt)