On a post, the young will be protected from most predators, but the box may get hot during a heat wave. On a building, whatever is below the box will probably get splattered with fecal matter. Don't put nest boxes above locations where vehicles or equipment is parked. Outside your bedroom window is not a good idea either, because the young owls can compete very noisily each time the adults return to the nest with food. In Great Horned Owl habitat, that is, in heavily wooded, riparian regions.
Great horns are the largest predator of the barn owl.
In the middle of your worst gopher-infested area. Owls prefer not to hunt in the area of their nest box so as not to attract the attention of potential predators. Multiple nest boxes in this vicinity will solve this problem because the hunting areas of the different nesting pairs will overlap. No, regular farm activity will not bother the owls as long as the box or the post is not bumped. In most cases they will remain quietly inside the nest box.