In case of lost password you can use another user in oneadmin group to change the lost password:

$ oneuser passwd --sha1 oneadmin 'newpassword'

Or

• Change the password directly in the database:
• Stop OpenNebula daemons (this is needed as some data is cached).
• Connect to the database and get the body of oneadmin user:
• select body from user_pool where name=’oneadmin’;

You will get an xml document with user data. The value you have to change is password. Make sure the password is hashed with sha1. You can do it with this command:

$ echo -n 'newpassword' | openssl sha1

Update the table with the edited xml:

update user_pool set body='<edited xml>' where name='oneadmin';

Now you can change your credentials and start OpenNebula again:

$ echo "oneadmin:new_password" > ~/.one/one_auth
$ one start

Src: http://serverfault.com/questions/545108/opennebula-sunston-user-oneadmin-password