A real time tool like Orocos RTT can not promise that your task is real time task.. It just means that you can use it to build real time task.
A simple case is that:
I have two task, one is period task which has lower priority than the other aperiodic task. If aperiodic need more process time than the interval time of period task, the periodic task can not run anymore as it can not get cpu time. So the periodic task is not periodic anymore. Of course ,for periodic task is not realtime. But if aperiodic task has highest priority in the OS, it will be realtime task.
So whether the task is realtime depends on how you design your multi-task. The real time tools like RTAI or RTLINUX ,hence orcos just give you the ability to build your realtime task.