OHSQ as warmup for SQ and FRSQ?
 

Any reason not to? Seems the lower body groove is the same and as long as the OHSQ weights are much less than the SQ or FRSQ weight it should be ok.

I've done it and found nothing too fatiguing about it though I didn't get too heavy and didn't go too high in my reps either. All I did was sub my warmup for squats as overhead squats and kept the weight the same. For example:

My warmup for squats is typically: 45 x 5, 95 x 4, 135 x 3, 185 x 2, 225 x 1 and then singles up until my working weight.

When I subbed in OHS I did it like this:
OHS - stick x 10, 45 x 5, 95 x 4, 135 x 3
BS - 185 x 2....etc

I think it's a good idea. It works very well for me.