It might be a good idea to point your IDE to the osv directory of your odoo installation (something like odoo/openerp/osv) as an external library. I never worked with PyCharm but in eclipse there is a menuitem like "Pydev - Pythonpath" under your project properties.Pointing this to the appropriate directory might help.

For development ease, it might also be a good idea to make a project of the Odoo branch, so you can easily look up any code in Odoo itself.