Here is an example:

I had to add a button in the orders view next to Reprint button.

View:

<openerp>

     <data>

         <record model="ir.ui.view" id="tpv_order_mail">

             <field name="name">pos.order.mail</field>

             <field name="model">pos.order</field>

             <field name="inherit_id" ref="point_of_sale.view_pos_pos_form"/>

             <field name="arch" type="xml">

                  <xpath expr="//field[@name='state']" position="before">

                         <button name="send_invoice" string="Send to..." type="object"/>

                  </xpath>

            </field>

        </record>

     </data>

</openerp>

The xpath is to select where the new element will be placed, so we tell the view to place the new button before the field named 'state'.

python:

We used an example to call the function of the new button:

@api.model

def send_invoice(self, context=None):

     return {

         'warning': {

            'title': "Toy op",

            'message': "Sepin el mandarin",

        }

    }

Hope this is useful.