When interacting with any API, whether in OneCart or elsewhere, there are normally two ways you can access data: Asking, and Receiving.
Asking for data (normally called "Polling") is when you set up your app to consistently request data from OneCart's API.
Receiving data can occur when the sending system (in this case, OneCart), notifies you of any updates. Webhooks are what are used to send data.
To add your webhook URL, log into your OneCart account and update the URL at the Edit Settings page: https://app.getonecart.com/edit_my_company
We provide a handy tool to quickly check whether your preferred webhook URL is working.
You can also use Webhook.site to test your webhooks for free (Recommended to make sure your receiving system is set up correctly).