We need to integrate with a system that exposes business entities by an OData co

I think that the Appian documentation is a good point to start with. See

forum.appian.com/.../Records_Tutorial

There is an example on who to access external data. I would prefer to use the JSON format as the XML parser in Appian does have trouble with some quite common things like namespaces and external XSDs.

That allows you to show data. Add some processes as related actions to modify it.

There are plugins to do HTTP requests as well as JSON processing. These two help you to parse the data when reading and to build the data structures when writing back.