This paper presents COBRA (Continuous Binary Re- Adaptation), a runtime binary optimization framework, for multithreaded applications. It is currently implemented on Itanium 2 based SMP and cc-NUMA systems. Using OpenMP NAS parallel benchmark, we show how COBRA can adaptively choose appropriate optimizations according to observed changing runtime program behavior. Coherent cache misses caused by true/false data sharing often limit the scalability of multithreaded applications. This paper shows that COBRA can significantly improve the performance of some applications parallelized with OpenMP, by reducing the aggressiveness of data prefetching and by using exclusive hints for prefetch instructions. For example, we show that COBRA can improve the performance of OpenMP NAS parallel benchmarks up to 68%, with an average of 17.5% on the SGI Altix cc-NUMA system.