This post describes the limitations of the OData endpoint in CRM 2011, which should still be relevant for CRM 2013 and 2015. There is no reason to believe that the OData endpoint will be developed further as Microsoft is working on a brand new WebApi to replace both the current OData endpoint and the SOAP endpoint.
Please refer to this article for general information on the available options when Qerying CRM using OData: OData system query options using the OData endpoint
With the release of Microsoft Dynamics CRM 2011, Microsoft have added a Windows Communication Foundation (WCF) data services (ODATA) endpoint. The endpoint facilitates CRUD operation on entities via scripts using Atom or Json format. This post mentions some of the considerations when using the endpoint, specifically around the use of retrieves.
First, the operations supported over this endpoint are limited to create, retrieve, update and delete. The REST philosophy does not support other operations and so we followed J. Microsoft did not implement others since the story around service operations was not fully developed in the current WCF data services offering.
The $format and $inlinecount operators are not supported. CRM’s OData endpoint only supports $filter, $select, $expand, $top, $skip, $orderby
Some of the restrictions when using the implemented operators are.
…/TaskSet?$expand=Contact_Tasks&$filter=Subject eq ‘test’ and Subject ne null
…/TaskSet?$expand=Contact_Tasks&$filter=Contact_Tasks/FirstName eq ‘123‘