The high-level procedure to create an external list in SP-O that uses a cloud-based WCF service is as follows:
1. Create a back-end LOB data source that you’ll run your service against;
2. Create a WCF service and deploy to Windows Azure;
3. Assess the permissions in the Metadata Store; and
4. Create an external content type (ECT) in SharePoint Designer that digests your WCF service and creates the list for you.