One of the key short-term objectives of the Synthetic Environment Core Architecture and Integration (SE Core A&I) program is integrating the U.S. Army's OneSAF Objective System (OOS) into the Close Combat Tactical Trainer (CCTT) and Aviation Combined Arms Trainer (AVCATT). To accomplish this, SE Core A&I must bridge conceptual, programmatic, and technology gaps between the impacted programs. SE Core's OOS integration activities will provide valuable lessons-learned regarding the transition of established, stable legacy systems to new, common technologies that are not specialized to their needs. This paper will focus on the technical and practical issues and successes to date with OOS integration into CCTT and AVCATT. Specific, detailed examples will be cited to illustrate the key lessons-learned, including transition from constructive to virtual needs, updates to OOS' Synthetic Natural Environment (SNE), transition of legacy models to a new architecture, impacts to fair-fight, and the spider web of impacts and dependencies reaching into other system components. Upcoming technical activities required to complete OOS integration will be described in detail, along with potential solutions.