From ONEWEB version 4.0.19.12, users can use JSON path syntax to retrieve data from REST services with complex JSON structures. For more information on JSON path syntax please check the site https://support.smartbear.com/alertsite/docs/monitors/api/endpoint/jsonpath.html.
Users can use this syntax on the output mapping screen for REST services under the RestDummyObject.
Users can use online JSON path evaluators to find the JSON expression to be used to retrieve a particular data from the JSON structure. There are multiple evaluators available online. For e.g. https://jsonpath.com/.