New article about how to import OData-based BDC Model in SharePoint Online
This article shows how to create an external content type (ECT) using the Northwind OData service provided at the OData.org site and how to import the extracted BDC model file in SharePoint Online to support a tenant-scoped external content type, which can be used in external lists.
Unfortunately SharePoint Designer 2013 is not supporting the creation of a BDC model based on OData sources. This is currently only possible by using Visual Studio and by extracting the model file from the App package file. The described technique below also applies to SharePoint Server 2013.