As I recall, the part of the strait that large tankers can actually use is very narrow. As a result, the tankers have to navigate the strait slowly and they are easy to intercept.

On top of that, Iran has several islands in perfect position to be used as naval bases. So, they can go our and put patrol boats (smaller, so not limited in where they can navigate, and well armed) in the way of ships, and threaten to sink them if they disobey orders to go no further.

As to why Oman doesn’t object, I can only assume that Oman doesn’t have enough naval power of its own to put up a serious fight against Iran. So, when the Iranians come and threaten tankers, whether it’s in international waters or Omani waters, Oman probably can’t do much about it.