We are stationed in Korea, about about an hour away from Osan (MH is Army). I don't have much information to offer since we have only been there once, but their Exchange is the best on the Peninsula, and nicest I have ever been to. The commissary is pretty average and Amazon will become your best friend if you like specialty products.
Most people from here go to Osan for shopping and custom made clothes. It's a little cheaper than where we are and we found a good tailor to make a suit for MH for dirt cheap.
Check out Facebook for specific groups. They were great for us when we were waiting to move. Sites in Seoul is a really good one and occasionally has info about Osan.