I'm pretty sure shorts are authorized as ACA, so long as they aren't excessively baggy/sagging, frayed, full of holes, PT/gym shorts, etc. Hell, they can even be short shorts (within good reason/not nut-huggers). ACA basically comes down to the simple rule of "If I have to ask, I probably can't wear it."
My NROTC unit specifically said no shorts when we reported for Freshman Indoc, so I'd probably go with pants or slacks for OCS pick-up. Then again, I'm just an NROTC puke.