Where in the world?
The flag of Oman has three horizontal stripes of white, red, and green with a red vertical stripe on the left containing the national emblem.

Oman

عمان

Population: 5,106,622

Region: Asia

Subregion: Western Asia

Capital: Muscat

Top Level Domain: .om

Currencies: Omani rial

Border countries

The flag of Saudi Arabia has a green field with white Arabic inscription and a sword.

Saudi Arabia

Population: 34,813,867

Region: Asia

Capital: Riyadh
The flag of United Arab Emirates has a horizontal tricolor of green, white, and black with a vertical red stripe at the hoist.

United Arab Emirates

Population: 9,890,400

Region: Asia

Capital: Abu Dhabi
The flag of Yemen has three horizontal stripes of red, white, and black.

Yemen

Population: 29,825,968

Region: Asia

Capital: Sana'a