Over.

When I was a kid, my parents mounted it over on the dispenser, and I would re-mount it under because the dispenser was placed high on the wall and, when pulled down sharply, the paper had less tendancy to keep rolling if mounted under. But my parents would always re-re-mount it over!

BTW my toilet is at an odd angle so, perforce, so must I be. In fact, it's at the opposite angle to the one in my last home. Felt a bit seasick when I first moved into the present one.