I saw alot of travellers speaking loudly and slowly to the locals, but this really isn't necessary, many people in Oman speak very, very good English, particularly in the souq (the Matrah one is amazing!) Don't be surprised if you are invited to sit down in a shop and talk with the owner, who more than likely will off you some fruitjuice or soda. If you are asked, accept if possible, it can be one of the most memorable moments of your trip!