Modern virtual machines often maintain multiple compiled versions of a method. An on-stack replacement (OSR) mechanism enables a virtual machine to transfer execution between compiled versions, even while a method runs. Relying on this mechanism, the system can exploit powerful techniques to reduce compile time and code space, dynamicallyde-optimize code, and invalidate speculative optimizations.This paper presents a new, simple, mostly compiler- independent mechanism to transfer execution into compiled code. Additionally, we present enhancements to an analytic model for recompilation to exploit OSR for more aggressive optimization. We have implemented these techniques in Jikes RVM and present a comprehensive evaluation, including a study of fully automatic, online, profile-driven deferred compilation.