Install ceiling fan on exposed conduit
If the current box is rated for a light fixture, then you can use it for a fan, if there is structure behind the box to attach the fan bracket to. You could use long wood screws to clamp the fan bracket to the box and the screws would go through holes in the box or beside the box and screw into a suitable wooden member in the ceiling or just in the OSB if it is thick enough.

You could not use small bolts into threaded holes in the box designed for a cover or for a light-weight light fixture. The new Hunter fans come with fairly long wood screws which are designed to hold the fan bracket in ceiling joists, but check that they are long enough. The instructions will state how much thread of these screws must be in solid wood.

You might have to build a split ring out of plywood or solid wood, place it around the electrical box and screw that to the OSB ceiling. Then the fan bracket would be secured to the ring with the special wood screws supplied with the fan. Modern fans have a large shroud which would obscure the ring. There would be a larger gap than usual between the top of the shroud and the ceiling, but I don't think it would look bad.