Hi, Add this in xml:

<record id="view_order_tree" model="ir.ui.view">
            <field name="name">sale.view_order_tree</field>
            <field name="model">sale.order</field>
            <field name="inherit_id" ref="sale.view_order_tree"/>
            <field name="arch" type="xml">
                <field name="state" position="after">
                    <field name="custom_field"/>
                </field>
            </field>
</record>

In py:

'custom_field': fields.char('Custom Field'),