Open Data Protocol, is an open protocol to allow the creation and consumption of queryable and interoperable RESTful APIs in a simple and standard way. All question related to OData in reference with Sitecore should use this tag.
You use the OData Item Service for read-only access to Sitecore items. The service authenticates requests using API keys. The service exposes the /sitecore/api/ssc/aggregate/content/ endpoint. It returns data in OData (JSON) format.