cooperative-market/purchase_order_generator/views/purchase_order.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <record id="view_purchase_order_form_inherit" model="ir.ui.view">
        <field name="name">purchase.order.form.inherit</field>
        <field name="model">purchase.order</field>
        <field name="inherit_id" ref="purchase.purchase_order_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='date_order']" position="after">
                <field name="original_cpo_id"
                       attrs="{'invisible': [('original_cpo_id','=',False)]}"/>
            </xpath>
        </field>
    </record>
</odoo>