odoo/addons/event_sale/event_sale_view.xml

<?xml version="1.0"?>
<openerp>
    <data>
        <record model="ir.ui.view" id="event_sale_order">
             <field name="name">event.product</field>
             <field name="model">product.product</field>
             <field name="inherit_id" ref="product.product_normal_form_view" />
             <field name="arch" type="xml">
                <div name="options" position="inside">
                    <field name="event_ok" on_change="onchange_event_ok(event_ok, context)"/>
                    <label for="event_ok"/>
                </div>
                <div name='ean' position="after">
                    <field name="event_type_id" attrs="{'readonly': [('event_ok', '=', False)]}"/>
                </div>
             </field>
        </record>

        <record model="ir.ui.view" id="event_order_line">
             <field name="name">event.sale.order</field>
             <field name="model">sale.order</field>
             <field name="inherit_id" ref="sale.view_order_form" />
             <field name="arch" type="xml">
                <xpath expr="//div[@name='discount']" position="after">
                    <label for="event" string="Event"/>
                    <div name="event">
                        <field name="event_id" colspan="3" class="oe_inline" domain="[('type', 'in', list(set((event_type_id,False))))]" attrs="{'invisible': [('event_ok', '=', False)],'required': [('event_ok', '!=', False)]}"/>                    
                    </div>
                    <field name="event_type_id" invisible="1"/>
                    <field name="event_ok" invisible="1"/>
                </xpath>
             </field>
        </record>
        
        <!-- the presence of 'event user' makes the order lines non-editable -->
        <record id="sale.view_order_form_editable_list" model="ir.ui.view">
            <field name="groups_id" eval="[(4, ref('event.group_event_user'))]"/>
        </record>        
        
    </data>
</openerp>