The square began to take shape as early as 1846, but only took its current name in 1862 when the political factions that were created after the removal of Otto took an oath of unity (omonoia in Greek) at this spot. In the beginning it was a small round square with a few luxurious houses. At the beginning of the 20th century, it developed as the center of the city's entertainment district and a meeting point for cosmopolitan Athenians. Gradually it would become a crossroads for all social strata, from all parts of the country, which is why it was characterized as the "navel" of Athens and the whole of Greece.

After World War II, the square began to lose its cosmopolitan character as it became a commercial and transportation hub and a field for political gatherings and conflict. The square has seen many changes and has recently been revived as lively meeting point. This rhythm is reflected by the continuous flow of water from its central fountain and the hydrokinetic sculpture by George Zongolopoulos known as the Pentacycle – the Five Circles.