Oman can be expensive. Many places accept cash only, especially outside of the malls and hotels, so it is best to have cash on hand.
Currency and Exchange
The national currency is the Omani rial (OMR), which maintains a fairly steady exchange rate against the U.S. dollar. The smaller denomination is called a baisa, which come in 100s. There are 1,000 baisas in 1 rial. There are 0.5 baisa coins as well. It is difficult to exchange U.S. currency for rials outside of the airport and some higher-end hotels.
ATMs and Banks
There are many banks in Oman, the largest of which are Bank Muscat, National Bank of Oman (NBO), Bank Dhofar, Bank Sohar, HSBC, Bank of Beirut, and National Bank of Abu Dhabi (NBAD). They all charge about the same fee for withdrawing money, so go with whatever you can find. However, ATMs are not as plentiful as they are in the U.S. They can always be found in airports and malls, but they may be difficult to find otherwise. Ask at your hotel about the location of the closest ATM.
Visa and Mastercard are more widely accepted than American Express, but all cards are usually taken at modern boutiques, hotels, and malls. Smaller shops, many restaurants, fast food places, and even some gas stations are cash-only.