Programming a distributed-memory parallel machine generally entails a high degree of complexity. Load balancing in particular is a demanding task. If high efficiency is to be maintained, this task cannot be solved by a distributed operating system alone, but must involve the application programmer. Instead of the underlying message-passing architecture being shielded from the programmer, it should be explicitly modeled. Three key concepts of a parallel operating system - dual, mobile and reactive objects - are presented. They provide simple but efficient mechanisms that can be easily utilized for such complex tasks as load balancing, i.e. initial placement and migration of application entities. To illustrate the applicability of these concepts, a simple VR application geoview - has been implemented on a message-passing architecture, and serves as an example throughout this paper.