In Muscat, where standard western clothing. There is little requirement for covering of arms or legs away from religious sites (mosques) and malls where you should cover shoulders and knees. Beach wear is standard beach wear.
Mosques require covering to wrists and ankles (some days you may get away with less, don't risk it if you don't want to be disappointed).
In the mountains/villages away from the capital, just be aware of yourself. Very few people would ever say anything (they're much too friendly, especially if you stop to chat) but they will be less used to western dress. Shorts and T shirt is fine. 4 months ago