framework needed for ceiling

[AstonHill]

So the outer walls of the room are aprox 20' by 8' and 3''. But the OSB sheets of timber are 8'X4' so two of them put together or one a side there is still going to be a 3" gap. Best Idea please for making the frame work up??? (and what type of timber to use) so I can screw my OSB stright onto the frame?

Thanks