As typical for the Arabian Peninsula, the best months to visit Oman are from autumn, through the winter and into spring when the temperatures are around a reasonable 20 to 30 degrees. In the summer months, temperatures are much hotter and it can feel quite humid. From May to September, the southern Dhofar region - which includes Salalah, experiences a monsoon known as the Khareef which lowers the temperature and turns this normally arid country lush and green.
Oman Climate Chart
Average Temperatures (°C) in Oman throughout the year