OpenERP/Odoo is a framework that do almost all the hard work in the behind and developer just do some definitions and arrangements. One of the things that this framework that do on behalf of  the developer is to create combo boxes. You just need to define the field that needs to be a combo box on the view as a many2one. 

Check, for example, the customer field on the sales order screen:

 'partner_id': fields.many2one('res.partner', 'Customer', readonly=True, states={'draft': [('readonly', False)], 'sent': [('readonly', False)]}, required=True, change_default=True, select=True, track_visibility='always'),