HT1194: Mac OS X Server v3: reset Open Directory administrator password - Not exactly
For this command:
mkpassdb -setpassword slot-ID
The only way I could make it work was to change it to:
mkpassdb -upo slot-ID
which prompted me for a password, and then spit out the hex code of the new password, which I replaced in Directory Utility.
iMac (27-inch, Late 2013), OS X Mavericks (10.9.1)