Nizwa is the largest city in Ad Dakhiliyah Region in Oman and was the capital of Oman proper. Nizwa is about 140 km from the Omani capit…Nizwa is the largest city in Ad Dakhiliyah Region in Oman and was the capital of Oman proper. Nizwa is about 140 km from the Omani capital Muscat. The population is estimated at around 83,544 people.