Wikipedia
OneAPI is a set of application programming interfaces (APIs) supported by the GSM Association that exposes over the Internet.
OneAPI, as defined by the telecom industry—led by GSMA and the Open Mobile Alliance (OMA)—is a set of standardized and lightweight Web-friendly application programming interfaces (API) for communications service providers (CSPs) to use to expose their networks. Building on OneAPI version 2.0 specifications, the current OneAPI specifications are at version 3 beta (published from February to April 2012).
Any mobile operator or service provider is able to implement and use OneAPI. OneAPI is intended to complement and not replace it, by providing access to network capabilities and information, regardless of operator.