You can create scheduler and in the method search the record with the domin condition dob is equal to current date month and day. If record find then send mail using mai.mail object.

Example,

To create the method in custom module:-

from openerp.osv import fields,osv
import datetime

class res_partner(osv.osv):
    _inherit = 'res.partner'

def send_birthday_email(self, cr, uid, ids=None, context=None):
        partner_obj = self.pool.get('res.partner')
        mail_mail = self.pool.get('mail.mail')
        mail_ids = []
        today = datetime.datetime.now()    
        today_month_day = '%-' + today.strftime('%m') + '-' + today.strftime('%d')    
        par_id = partner_obj.search(cr, uid, [ ('customer','=',True),('birthdate','like',today_month_day)])
        if par_id:
            try:
                for val in partner_obj.browse(cr, uid, par_id):
                    email_from = val.email
                    name = val.name                
                    subject = "Birthday Wishes"
                    body = _("Hello %s,\n" %(name))                 
                    body += _("\tWish you Happy Birthday\n")                     
                    footer = _("Kind regards.\n")         
                    footer += _("%s\n\n"%val.company_id.name)
                    mail_ids.append(mail_mail.create(cr, uid, {
                        'email_to': email_from,
                        'subject': subject,
                        'body_html': '<pre><span class="inner-pre" style="font-size: 15px">%s<br>%s</span></pre>' %(body, footer)
                     }, context=context))
                    mail_mail.send(cr, uid, mail_ids, context=context)            
            except Exception, e:
                print "Exception", e    
        return None

In the xml File Create Scheduler

 <record forcecreate="True" id="ir_cron_employee_birth" model="ir.cron">
            <field name="name">Employee Birthday scheduler</field>
            <field eval="True" name="active"/>            
            <field name="interval_number">1</field>
            <field name="interval_type">days</field>
            <field name="numbercall">-1</field>
            <field eval="False" name="doall"/>
            <field eval="'res.partner'" name="model"/>
            <field eval="'send_birthday_email'" name="function"/>
            <field eval="'()'" name="args"/>
        </record>

Or

In Openerp using Menu Setting - > Configuration -> Scheduler -> Scheduler Action Create scheduler.