Use below code,

In PYTHON file:

class SaleOrderLine(models.Model):
    _inherit = 'sale.order.line'
    qty_available = fields.Float(related='product_id.qty_available', string="Qty Available", readonly=True)

In XML file:

     <record id="view_order_form_inherit" model="ir.ui.view">
            <field name="name">sale.order.form.inherit</field>
            <field name="model">sale.order</field>
            <field name="inherit_id" ref="sale.view_order_form"/>
            <field name="arch" type="xml">
                <data>
                     <xpath expr="//field[@name='order_line']/form/group/group/field[@name='price_unit']" position="before">
                         <field name="qty_available"/>
                    </xpath>
                    <xpath expr="//field[@name='order_line']/tree/field[@name='price_unit']" position="before">
                         <field name="qty_available"/>
                    </xpath>
                </data>
           </field>
        </record>