UMTS networks will allow the deployment of new types of applications, that are rich, interactive and multimedia capable, because of increased bandwidth, richer devices and location awareness. These applications are not necessarily provided by the network operator, but can be created and deployed by any third party. However, these third parties will need access to the UMTS core network capabilities, especially when it comes to call related services, location based services and services that charge for certain content. Intelligent Network (IN) based service creation and delivery platforms deployed in current 2G and 2.5G networks do not offer suitable interfaces for this, since third parties need to have detailed knowledge and expertise of low level, telecommunication specific protocols and can jeopardize the integrity of the network. The recent activities within the standardization bodies 3GPP and ETSI SPAN, and the industry forums Parlay and JAIN show an increased interest in the area of open network Application Programming Interfaces (APIs). The Open Service Access (OSA) specification is a collection of open network APIs for UMTS application provisioning defined by 3GPP. OSA is meant to allow third party application development and deployment by means of open, secure, and standardized access to core network capabilities, while preserving the integrity of the underlying network. We describe a prototype implementation, and list the possibilities and limitations of the OSA specification.