sale_order_barcode/views/sale_order_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>

        <record id="sale_order_view_form" model="ir.ui.view">
            <field name="name">Sale Order Barcode Inherit Form</field>
            <field name="model">sale.order</field>
            <field name="inherit_id" ref="sale.view_order_form"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='order_line']" position="before">
                    <button name="%(sale_order_barcode_action)d" type="action"
                        string="Lector" states="draft"/>
                </xpath>
            </field>
        </record>

    </data>
</openerp>