Event-driven operating systems such as TinyOS are the preferred choice for wireless sensor networks. Alternative designs such as MANTIS following a classical multi-threaded approach are also available. Event-based systems are generally more energy efficient than multi-threaded systems. However, multi-threaded systems are more capable than event-based systems of supporting time critical tasks as task preemption is supported. Timeliness can be traded for energy efficiency by choosing the appropriate operating system. In our recent work we have shown that the multi-threaded system MANTIS can be modified to be as energy efficient as TinyOS. As a result, the modified MANTIS can be used to fit both sensor network design goals of energy efficiency and timeliness. This solution is not considered optimal as most existing sensor network applications and software libraries are developed for TinyOS. Therefore, we present a TinyOS modification that adds preemption while retaining the existing TinyOS structure and features.