Once a major derby for the power of the Piraeus city, especially pre WWII, but since Olympiacos' dominance in the 90's and afterwards along with the steady decline of Ethnikos, has led to the derby losing importance over the last 25 years in football, however the rivalry still exists in water polo.

Olympiacos traditionally represented the mass and lower classes of the city of Piraeus and its suburbs while Ethnikos, in contrary, represented the upper class of the city of Piraeus and not much of its suburbs.

were more or less evenly matched and had great battles for Piraeus supremacy in the 1920s and 1930s,[4] but thereafter Olympiacos became increasingly more powerful and successful and began to pass Ethnikos by.

In 1956, Olympiacos won the championship with Ethnikos finishing second meaning the first and only time in which the top two positions in the league were occupied by Piraeus's teams.

Along the way, Olympiacos developed a habit for luring Ethnikos' best players, like Greece men's national football team players Philippos Kourantis in the late 1920s, Giannis Chelmis in the late 1930s and Giannis Ioannou in the early 1950s.

[5] This habit continued in the later decades with Christos Arvanitis, Tasos Mitropoulos, Stavros Papadopoulos, Thomas Rohrbach.

[7] Olympiacos and Ethnikos are the greatest clubs in the history of men's Greece water polo.

After WW2, both clubs remained a dominant force along with NO Patras, fighting for the title almost every year.

The Greek cup that started in 1953 was abolished in 1958 due to unprecedented dominance of Ethnikos who had achieved six straight doubles in 1958.

Olympiacos had great difficulty in beating rival Ethnikos, let alone winning the championship.