The best time to visit Oman is October to March. Temperatures along the coast drop to a pleasant 20–25°C during the day, cool enough for hiking, fort-visiting, and driving the wadis comfortably. Nights can get chilly in the mountains — bring a light jacket if you're heading up to Jebel Akhdar or Jebel Shams.
Summer (May to September) is brutal on the northern coast, with daily highs regularly exceeding 40°C and humidity spiking sharply in June along the sea. Everything is air-conditioned, so if you're mainly moving between hotels and cars it's survivable, but outdoor sightseeing becomes genuinely unpleasant. That said, if your aim is Salalah in the south, summer is actually the season: the khareef monsoon rolls in from the Indian Ocean between June and September, turning the Dhofar hills green and misting the coastline. Temperatures stay around 25–30°C. It's one of Arabia's stranger and more appealing anomalies.
Oman gets very little rain in the north. When it does come, it tends to arrive in short, intense downpours that cause flash floods in the wadis. If you're camping in or driving through a wadi and it starts raining — even distantly — get out. Flash floods move fast and with very little warning.
Ramadan shifts each year; during it, eating and drinking in public during daylight hours is frowned upon, many restaurants only open after sunset, and the pace of life slows noticeably. If you're visiting during Ramadan, go with it rather than against it — the iftar (breaking of the fast) evenings are lively and generous.