hello,

here is an example to add a field to sale order line :

py file

_inherit = 'sale.order.line'

my_field = fields.Many2one('some.class', string="Any Name")

xml file

<record model="ir.ui.view" id="sale_form_view_extends">

            <field name="name">my.name</field>

            <field name="model">sale.order</field>

            <field name="inherit_id" ref="sale.view_order_form"/>

            <field name="arch" type="xml" >

                <xpath expr="//notebook/page/field[@name='order_line']/tree/field[@name='product_uom_qty']" position="after">

                    <field name="my_field"/>

                </xpath>

            </field>

        </record>

Hope this helps.If you need more help, please post your code.