The response from "GET /api/v2/provisioning_templates/Kickstart default/os_default_templates" shows the following fields:

    {
      "id": 3,
      "provisioning_template_id": 4,
      "provisioning_template_name": "Kickstart default",
      "template_kind_id": 3,
      "template_kind_name": "provision",
      "config_template_id": 4,
      "config_template_name": "Kickstart default" 
    }

This makes sense if you're using the nested route under /operatingsystems, but it should include the operatingsystem ID in case you're accessing it under templates.