pos_buttons/views/purchase_order_button_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data> 
        <!-- Purchase Order Tree -->
        <record id="purchase_order_button_tree" model="ir.ui.view">
            <field name="name">pos_buttons.purchase.order.button.tree</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_tree"/>
            <field name="arch" type="xml">
                <xpath expr='//tree[@string="Purchase Order"]' position='attributes'>
                    <attribute name="create">false</attribute>
                </xpath>
            </field>
        </record>
        
        <!-- Purchase Order Form -->
        <record id="purchase_order_button_form" model="ir.ui.view">
            <field name="name">pos_buttons.purchase.order.button.form</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form"/>
            <field name="arch" type="xml">
                <xpath expr='//form[@string="Purchase Order"]' position='attributes'>
                    <attribute name="create">false</attribute>
                </xpath>
                <xpath expr='//form[@string="Purchase Order"]' position='attributes'>
                    <attribute name="edit">false</attribute>
                </xpath>
            </field>
        </record>    
    </data>
</openerp>