If you can start your openerp server then password is in config file,

Look at /etc/openerp-server.conf (or some custom location) open the file and read the ass (db_password = some_pass)

Otherwise, if you can't run openerp server, then you will first need to change the password, 
(and write new one in config file ,restart server)
if you need to change pass:
psql
alter role openerp with password 'your_new_password';

hope it helps