server-tools/base_cron_oneshot/views/ir_cron.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

  <record id="ir_cron_view_form" model="ir.ui.view">
    <field name="name">ir.cron.form</field>
    <field name="model">ir.cron</field>
    <field name="inherit_id" ref="base.ir_cron_view_form"/>
    <field name="arch" type="xml">
      <field name="state" position="after">
        <field name="oneshot"/>
      </field>
      <field name="numbercall" position="attributes">
        <attribute name="attrs">
          {'readonly': [('oneshot', '=', True)]}
        </attribute>
      </field>
    </field>
  </record>

</odoo>