I like OSB vs MDF idea and yes definitely use drywall to finish. Like Rives says you don't want to use too much rigidty and have the room not absorb.
If you have some specific question with detail I am able to give you some direction based on room dimensions and detail built into the walls. I will even do a basic RT60 calculation for your room with and without OSB/MDF and chart the axial mode that can be plotted for comparison against ideals. No room is perfect even after extensive planning and some tweaks are necessary. If your room is for HT the demand is less stringent because almost everything in HT is fake and loose, kinda like golden topping.
email me if you care to discuss
If you have some specific question with detail I am able to give you some direction based on room dimensions and detail built into the walls. I will even do a basic RT60 calculation for your room with and without OSB/MDF and chart the axial mode that can be plotted for comparison against ideals. No room is perfect even after extensive planning and some tweaks are necessary. If your room is for HT the demand is less stringent because almost everything in HT is fake and loose, kinda like golden topping.
email me if you care to discuss