it is not best idea to install openerp in ubuntu from .rpm, while there is 'deb package; I would suggest to use .deb one.

on the other hand, if because of some reasons you want to user .rpm, install first rpm in ubuntu (apt-get install rpm), and then install .rpm package. no matter which package you will chose, after installation you would need to configure postgresql to be ready for first use/configuration of openerp (installation in linux is not 'out-of-the-box' like in windows)

regards

Tomasz