A growing number of smart cities and Administrations are inclined to collaborate and mutualize their efforts and resources for IoT deployments and open data sharing. This Recommendation intends to study the concept and potential of developing a secured open and interoperable API in the context of IoT deployment and open data management in smart cities. It will analyse current solutions implemented by Administrations around the world, where applicable, including those adopted by smart cities, to share their data through open and interoperable interfaces. It will subsequently specify an open and interoperable API for secured Open Data architecture as well as to support IoT data interoperability for smart cities. This Recommendation presents a complete set of Open APIs dedicated to smart cities offering different features covering the needs of interoperable smart city framework development. In order to achieve interoperability among heterogeneous platforms and development of smart cities, the Recommendation proposed "interoperability points" in southbound and northbound in smart city framework. It provides a list of core API sets focusing on data interoperability, including context data management APIs, data transactions APIs, data storage APIs, and security APIs. Through the mechanism of subscriptions, it is possible to get a performant and scalable context data managment. The data storage APIs allow a granular management of the saved data for all cases, in particular both for open data and private data. The data transaction APIs facilitate exposure and access to the data through a data marketplace. In addition, security and privacy APIs are seriously taken into account to provide secure data exchange. It should be noted that data interoperability with open APIs can be completed with using common data models which is briefly discussed. Common data models built upon the collaboration with several standard fora and European projects are opened to public to use of them. The development of interoperable framework makes smart city platforms cost-efficient, flexible and extendable. The interoperability is not a choice but a must in smart city systems that embeds multiple verticals.