$ ls -l
total 8
drwxrwx--- 2 foo  foo 4096 Jun  3 16:01 a
drwxrwxr-x 3 root foo 4096 Jun  3 16:02 b

$ rsync -av  -O  --delete a/ b/
sending incremental file list
rsync: failed to set permissions on "/tmp/r/b/.": Operation not permitted (1)
deleting 2
./
1

sent 115 bytes  received 138 bytes  506.00 bytes/sec
total size is 0  speedup is 0.00
rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1196) [sender=3.1.2]