If there are other guard skills you want to train, you can have a rhythm of 2 weeks OD-1 week something else-2 weeks OD-1week something else...
If you don't stick with OD for now, and once you're finished with it, buy some big man trainees and rotate the three big man main skills (REB, ID, IS)
Switching assignments is advantageous if you train several related skills (i.e. JR, PA and OD, or IS, REB and ID, or similar combinations) to optimize the elastic training effect. When training one single skill up, it's better to stick to that one constantly though.