The precise location of Opis has not been established, but from the Akkadian and Greek texts, it is clear that it was situated on the east bank of the river Tigris, not far from the confluence of Tigris and Diyala. A possible candidate is a place called Tulûl al-Mugaili.
Opis is mentioned for the first time at the beginning of second millennium BCE. In the fourteenth century, it became the capital of an administrative region in Babylonia. At an unknown moment, the Babylonians dug the "royal canal" between the Euphrates and the Tigris, which ended near Opis. The Babylonian king Nebuchadnezzar built a wall between the two rivers to protect his country from a Median invasion. This wall also ended near Opis.
Opis was also situated on the Royal road, which connected Elam's capital Susa to the Assyrian heartland and - later - the Lydian capital Sardes.
In October 539 BCE, Opis was the theater of an important battle. The Babylonian king Nabonidus defended his country against the Persians, who were commanded by their king Cyrus the Great. The Babylonians were defeated and the native population revolted against its government. Without further fighting, Cyrus marched to and captured Babylon. The contemporary Nabonidus Chronicle describes the event:
In the month of Tashritu, when Cyrus attacked the army of Babylonia in Opis on the Tigris, the inhabitants of Babylonia revolted, but he [Cyrus, Nabonidus?] massacred the inhabitants. On the fifteenth day [12 October], Sippar was seized without battle. Nabonidus fled. On the sixteenth day, [the Persian commander] Gobryas, the governor of Gutium, and the army of Cyrus entered Babylon without battle. Afterwards, Nabonidus was arrested in Babylon when he returned there.
Two centuries later, Opis was the place where the European soldiers of the Macedonian conqueror Alexander the Great revolted (August 324 BCE). They felt neglected because Alexander seemed to have given equal rights to the Persians. You can read more about this mutiny over here.
At the end of the fourth century BCE, king Seleucus I Nicator, the successor of Alexander, built Seleucia on the opposite bank of Opis. From now on, Opis was a mere suburb. The Roman historian Tacitus informs us that in the first century, Greek and native inhabitants were still recognizable and had institutions of their own. The Parthians, who had taken over the country in the second century BCE, had hardly any influence.
In the second century CE, the Parthians moved the center of this city to the eastern bank again, and founded Ctesiphon near Opis.