Updated since 1.0.7
In OMGPI each OMGPlayer has own displayObjective in scoreboard - shown in sidebar. To control it in OMGPI, it is recommended to use ObjectiveBuffer - class made for this task. It works like drawing in OpenGL: you should first fill in buffer and then load it in the objective. For that, you first create an objective with new ObjectiveBuffer() or new ObjectiveBuffer(existingStringList). In first case, you can do buffer.lines.add(string) to add a new line. Then you could do buffer.loadInto(player.displayObjective) - and lines you've created will appear on sidebar for that player. Because of Minecraft limitations, sidebar can have maximum size of 16 lines. If you are reusing a buffer, do buffer.lines.clear so you can always buffer.lines.add() a new line.