oca-technical/odoo-bringout-oca-rma-rma_sale/rma_sale/views/report_rma.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <template id="report_rma_document" inherit_id="rma.report_rma_document">
        <xpath expr="//tr[@name='tr_picking']" position="before">
            <tr t-if="doc.order_id" name="tr_picking">
                <td>Sale order</td>
                <td>
                    <span t-field="doc.order_id" />
                </td>
            </tr>
        </xpath>
    </template>
</odoo>