Can I use OSB or plywood instead of 1x6 boards to hang metal panels?
Yes, OSB or plywood would be a suitable substructure. It will distribute the load and create a stable mounting surface for your assembly. OSB is similarly strong to plywood and can be  more uniform, depending on the grade of plywood. Its drawback is its susceptibility to swelling and flaking due to repeated moisture exposure.

Individual boards are less solid as a system and you may end up with concentrated loads. That said, if they're mounted solidly, they could carry substantial weight as well. 

The key is that the anchors you use in the substructure need to be adequate to grab sufficiently in only 1/2" of material. Larger diameter screws substantially increase pullout strength.