Army Capabilities Integration Center (ARCIC) is integrating OneSAF into its Battle Lab Collaborative Simulation Environment (BLCSE), in an ongoing effort to integrate new technologies into war gaming experimentation. This integration requires modifications to OneSAF's High Level Architecture (HLA) interoperability module to enable OneSAF to interact in the experimental and ever-changing BLCSE federation. This paper introduces OneSAF HLA interface design concepts and provides detailed examples to allow the reader to perform similar development in his own simulation environment. The reader will learn about the OneSAF HLA interoperability architecture and structure, how to use OneSAF's Object Data Model (ODM) and Federation Object Model (FOM) variables, and how to implement converter classes when FOM changes are required. To illustrate these concepts, the paper uses case studies from the BLCSE integration effort, which has resulted in successful, record-run experiments using OneSAF in HLA mode. Specifically, the case studies describe how to implement a new FOM attribute to enable the OneSAF Plan View Display (PVD) to display whether an external individual combatant is armed, and how to implement a new FOM interaction to perform terrain damage effects.