Map from Google - Oman

In Oman, the climate is tropical desert almost everywhere, with some summer rains in both the northern and southern mountain areas, and clouds brought by the summer monsoon along the eastern coast. The monsoon, from mid-June to mid-September, is due to wet currents blowing towards the Asian continent, and brings limited effects in terms of rain, but it's nonetheless able to influence the climate of the country.
Let's now see the climatic zones.

The north coast

The north coast

Along the coast of the Gulf of Oman, the climate is desert and sunny all year-round, with a few winter rains, more likely in the far north, ie the Musandam Peninsula, an exclave of Oman in the United Arab Emirates, where 15/20 millimetres (0.6/0.8 inches) of rain per month fall from December to March. This area is very hot in summer, with highs around 37/38 °C (99/100 °F) on average, and the humidity of the sea that makes the heat oppressive. When the wind blows from inland the humidity drops, but the temperature can approach as high as 50 °C (122 °F). The winter temperature is pleasant: the daily average is around 20 °C (68 °F) in the far north, and 21 °C (70 °F) in Muscat, the capital. In contrast to the northern tip, in summer, along the northern coast (called Al Batinah) where Muscat is located, the hottest month is June, because in July and August, cloud banks brought by the southwest monsoon can slightly lower the daytime temperature, which passes from 40 °C (104 °F) in June to 38 °C (100 °F) in July and 36 °C (97 °F) in August.
Here are the average temperatures of Muscat.
Average temperatures - Muscat
MuscatJanFebMarAprMayJunJulAugSepOctNovDec
MuscatJFMAMJJASOND
Min (°C)171820242930302827242118
Max (°C)252630354040383636353027
Min (°F)636468758486868281757064
Max (°F)777986951041041009797958681

In Muscat the rains are very scarce, given that they amount to 100 millimetres (4 inches) per year, and in the summer, despite the southwest monsoon, it almost never rains. Here is the average precipitation.
Average precipitation - Muscat
MuscatJanFebMarAprMayJunJulAugSepOctNovDecYear
MuscatJFMAMJJASONDY
Prec. (mm)22271511512500510103
Prec.(in)0.91.10.60.40.200.10.2000.20.44.1
Days23210111000213

The Gulf of Oman is warm enough for swimming all year round, and it becomes very warm in summer, as we can see in the following table.
Sea temperature - Muscat
MuscatJanFebMarAprMayJunJulAugSepOctNovDec
MuscatJFMAMJJASOND
Sea (°C)242424272931313029292725
Sea (°F)757575818488888684848177

Muscat

The Hajar Mountains

Hajar Mountains

In the mountains located near the northern coast, called Al Hajar, and culminating in Jebel Shams, 3,009 metres (9,872 feet) high, the south-west monsoon brings a bit of rain, which added to the relatively poor rains (and sometimes even snowfalls) of winter, raises the total of the year in some areas above 200/300 mm (8/12 in), still not much, but sufficient for the growth of some shrubs or drought-resistant plants. In Saiq, at almost 2,000 metres (6,500 feet) above sea level, the average temperature in January is 10 °C (50 °F), so it is quite low, with possible frosts at night. In June and July, highs are around 29/30 °C (84/86 °F). On the contrary, at lower altitudes the summer heat is intense.
Average temperatures - Saiq
SaiqJanFebMarAprMayJunJulAugSepOctNovDec
SaiqJFMAMJJASOND
Min (°C)57101418202120171396
Max (°C)151820242931313028242017
Min (°F)414550576468706863554843
Max (°F)596468758488888682756863

In Saiq, 310 millimetres (12 inches) of rain per year fall, with a relative maximum in summer. Here is the average precipitation.
Average precipitation - Saiq
SaiqJanFebMarAprMayJunJulAugSepOctNovDecYear
SaiqJFMAMJJASONDY
Prec. (mm)203035302015455030151010310
Prec.(in)0.81.21.41.20.80.61.821.20.60.40.412.2
Days23322344211127

Even in the Musandam Peninsula, there are mountains (the small circle at the top of the map), whose highest point is Jebel Al Harim, 2,087 metres (6,847 feet) high, but they are desert, because in summer they do not receive rainfall.

The east coast

Coast of the Arabian Sea

Along the coast of the Arabian Sea the climate is desert; moving south, winter gets progressively warmer, while summer becomes less hot, but also more humid and cloudy, due to the greater influence of the south-west monsoon (called Khareef). A moderate wind often blows, making the sea choppy. However, there can be periods, that typically last two or three days, in which the wind blows from inland, causing sudden increases in temperature, especially from April to July, with peaks above 40 °C (104 °F), often accompanied by sandstorms; these events are more likely in the northern part of the coast, while in the south, the coast of Dhofar is more sheltered from these hot winds.
In the island of Masirah, the temperature is high even in winter; in this season there is little rain and the sun shines. The temperature gradually increases until April, May and June, when it reaches 35 °C (95 °F), then it moves back to 31/32 °C (88/90 °F) in July and August, when humidity and cloudiness increase, even though the rains remain scarce. In a typical year, just 40 mm (1.6 in) of rain fall.
Average temperatures - Masirah
MasirahJanFebMarAprMayJunJulAugSepOctNovDec
MasirahJFMAMJJASOND
Min (°C)191921242626242324232321
Max (°C)272830343635323131323028
Min (°F)666670757979757375737370
Max (°F)818286939795908888908682

The effects of the monsoon are more evident in the southern part of the coast. In Salalah, the capital of Dhofar, the daytime temperature reaches 32/33 °C (90/91 °F) in May and June, but then it goes down to 27/29 °C (81/84 °F) in July and August, when even a bit of rain falls, just 20/25 mm (0.8/1 in) per month, in the form of frequent drizzle. In addition, in July and August a permanent layer of clouds and fog hangs over this part of the coast; for this reason the daily temperature range is narrow.
Average temperatures - Salalah
SalalahJanFebMarAprMayJunJulAugSepOctNovDec
SalalahJFMAMJJASOND
Min (°C)192022242727252424232220
Max (°C)282830323332292729313129
Min (°F)666872758181777575737268
Max (°F)828286909190848184888884

Here is the average precipitation in Salalah. Although in July and August there are "officially" only 7/8 days of rain per month, days with light drizzle occur more often.
Average precipitation - Salalah
SalalahJanFebMarAprMayJunJulAugSepOctNovDecYear
SalalahJFMAMJJASONDY
Prec. (mm)10668822251050596
Prec.(in)000.20.20.30.30.910.40.200.23.8
Days02011378300025

The Arabian Sea is warm enough for swimming all year round, and it is warmer in spring and autumn, while in summer the water temperature drops a bit, because the wind increases evaporation and mixes the water layers.
Sea temperature - Salalah
SalalahJanFebMarAprMayJunJulAugSepOctNovDec
SalalahJFMAMJJASOND
Sea (°C)252526282928252425282726
Sea (°F)777779828482777577828179

The southern mountains

Southern mountain range

In the southern coastal range of the Dhofar region, north of Salalah, the climate is cooled by altitude, and the summer monsoon is able to discharge a bit of rain, so that the landscape is green, at least along the slopes facing southwest and during the rainy season. At 700/1,000 metres (2,300/3,300 feet) above sea level, in winter the temperatures are mild, around 21/22 °C (70/72 °F) during the day, and similar in summer, while in spring and autumn they are higher, around 28/30 °C (82/86 °F). In summer, the rains are usually light, but they occur almost daily.
Here are the average temperatures of Qairoon Hairitti, located at 850 metres (2,800 ft) above sea level.
Average temperatures - Qairoon
QairoonJanFebMarAprMayJunJulAugSepOctNovDec
QairoonJFMAMJJASOND
Min (°C)131417202121201919191714
Max (°C)212325293029232126282622
Min (°F)555763687070686666666357
Max (°F)707377848684737079827972

Dhofar mountains

The desert

Desert climate zone

In the vast inland desert area, which covers most of the country, the climate is desert, mild in winter (with cool nights and warm days), and torrid in summer, with average highs regularly above 40 °C (104 °F), but with peaks of as high as 50 °C (122 °F). However, in summer some cloudiness can form here as well, and there's a slight decrease in maximum temperatures in July and August, but generally it doesn't rain.
Here, for example, the average temperatures of Haima, located in the center of the country.
Average temperatures - Haima
HaimaJanFebMarAprMayJunJulAugSepOctNovDec
HaimaJFMAMJJASOND
Min (°C)121417222527282524211815
Max (°C)273033394344424240373329
Min (°F)545763727781827775706459
Max (°F)818691102109111108108104999184

Every now and then, from May to November, Oman can be reached by tropical cyclones, which bring gale force winds and torrential rainfall, as happened with cyclone Keila on early November 2011.

When to go

The best time to visit Oman, to make excursions in the desert, in cities and in mountains, is winter, from December to February, which is sunny, pleasantly warm on the coast, and quite warm in the desert during the day, but cool at night, and even cold in the mountains.
For a beach holiday, as we have seen the sea is warm all year round; the sea temperature is good even in the period from November to March, but in November there is still some risk of cyclones on the coast of the Arabian Sea, while in January the temperature can sometimes be a bit cool in the coast of the Gulf of Oman, at least for those who are sensitive to cold.

What to pack

In winter: in Muscat and the north, light clothes for the day, sweater and jacket for the evening and cool days, scarf for the wind. For high mountains, spring/autumn clothes, sweater and jacket for the evening. For the desert, light clothes for the day, scarf or turban, desert boots or sandals, sweater or jacket for the evening and early morning. For the Arabian Sea coast (see Salalah and Masirah), light clothing, a sweatshirt for the evening. For the southern coastal mountain range, light clothes for the day, sweater and jacket for the evening. For the reef, snorkeling equipment, water shoes or rubber soled shoes.

In summer: in Muscat, the north and the desert, loose fitting, lightweight clothes, made of natural fabric (cotton or linen), desert turban, sweater and scarf for air conditioned places. For high mountains, light clothes, sun hat, a sweatshirt for the evening. For the Arabian Sea coast (see Salalah and Masirah), light clothes, light sweatshirt and scarf for the breeze, light raincoat. For the southern coastal mountain range, spring/autumn clothes, a light jacket and a sweatshirt, raincoat.
For women, it is better to avoid shorts and miniskirts outside tourist resorts.