El Oro is one of 125 municipalities in the State of Mexico within the United Mexican States. The municipal seat is the town of El Oro de Hidalgo. The name El Oro means "the gold" in Spanish. It has subsequently been given an alternative Nahuatl name of "Teocuitlatl", meaning "sacred excrement". Its seal, in the form of an Aztec glyph, contains elements referring to gold and to caves, of which there are many in the municipality.

Mexico is a state in central Mexico that is divided into 125 municipalities. According to the 2020 Mexican census, it is the most populated state with 16,992,418 inhabitants and the 8th smallest by land area spanning 22,351.8 square kilometres (8,630.1 sq mi).