orden_servicio/views/sale_order.xml

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

        <record id="view_sale_order_form_inherit" model="ir.ui.view">
            <field name="name">sale.order.form.inherit</field>
            <field name="model">sale.order</field>
            <field name="inherit_id" ref="sale.view_order_form"/>
            <field name="arch" type="xml">
            <!-- <form> -->
                <field name="order_line" position="after">
                  <field name="state" invisible="1"/>
                  <button name="open_order_service_form" string="Crear Orden de Servicio" type="object" class="oe_highlight" attrs="{'invisible': [('state', '!=', 'sale')]}"/>
                </field>
            <!-- </form> -->
            </field>
        </record>
    </data>
</openerp>