In openerp first install account module and inherit the views of account module and pass the id of account module views form xml file then give reference in your own module and add your new fields in xml file.
Use this code in your own module in xml file,

<record id="view_account_order_extend1_form" model="ir.ui.view">
    <field name="name">account.order.form.price.extend</field>
    <field name="model">account.invoice</field>
    <field name="inherit_id" ref="account.invoice_form" />
    <field name="arch" type="xml">
        <xpath expr="//page[@string='Invoice Lines']/field[@name='invoice_line']/tree[@string='Invoice Lines']/field[@name='quantity']" position="before">
            <field name="maintenance_Plan"/>
        </xpath>
    </field>
</record>