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.

https://doc.sitecore.net/sitecore_experience_platform/developing/developing_with_sitecore/sitecoreservicesclient/the_odata_item_service

https://doc.sitecore.net/sitecore_experience_platform/developing/developing_with_sitecore/sitecoreservicesclient/api_keys_for_the_odata_item_service