 Muscat residents evacuated as storm clouds gathered |
Tropical Cyclone Gonu is continuing north-west towards Iran but wind speeds have dropped to about 100mph (175 km/h).
Thousands of residents were evacuated from Oman's coast and the offshore island of Masirah. Hundreds have also been evacuated from the Iranian coast.
The storm is the most powerful in the region for 60 years.
The US military predicted that Gonu would become a "very weak" tropical cyclone over the next 48-72 hours.
But there were predictions that heavy rains could cause flash flooding by the time Gonu reached the south coast of Iran.
High waves
In Oman, people were told to stay indoors, while schools and public buildings were emptied to make room for the evacuees.
BBC Gulf correspondent Julia Wheeler says Gonu is bringing both strong winds and waves of up to 12m high (36ft).
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In the coastal capital of Muscat, people have been advised to stay indoors and to switch off their power.
Schools have been designated as emergency shelters and the international airport is closed.
The most powerful part of the storm was expected to hit Oman on Thursday, before moving north across the Gulf to Iran.
Hundreds of people living in low-lying areas of Iran have been moved to higher ground amid warnings that the waning storm could bring heavy rain, flash floods and high waves.
An emergency official quoted by the AFP news agency said all people living close to the sea between the towns of Chahbahar and Konarak - a 25km (15 miles) stretch of coast - had been evacuated.
Reports say Cyclone Gonu is the strongest to hit the Arabian Peninsula since records started in 1945.
While crossing the Indian Ocean on Monday it reached the equivalent of a Category Five hurricane - the highest category available.
