fields_security/views/purchase_order.xml

<?xml version="1.0"?>
<openerp>
    <data>
        <record id="purchase_form_view" model="ir.ui.view">
            <field name="name">fields_security.purchase.form</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form"/>
            <field name="groups_id" eval="[(6, 0, [ref('group_restrict_fields')])]"/>
            <field name="arch" type="xml">
                <xpath expr="//form/sheet/notebook//page[@string='RFQ &amp; Bid']" position="attributes">
                    <attribute name="attrs">{'invisible':True}</attribute>
                </xpath>
                <field name="minimum_planned_date" position="attributes">
                    <attribute name="attrs">{'invisible':True}</attribute>
                </field>
                <field name="fiscal_position" position="attributes">
                    <attribute name="attrs">{'invisible':True}</attribute>
                </field>
                <field name="partner_ref" position="attributes">
                    <attribute name="attrs">{'invisible':True}</attribute>
                </field>
                <xpath expr="//field[@name='order_line']/tree//field[@name='date_planned']" position="attributes">
                    <attribute name="invisible">1</attribute>
                </xpath>
            </field>
        </record>
    </data>
</openerp>