It's not easy to define opa. The word is flexible and has taken on many new meanings. Traveling in Greece or just exploring Greek popular culture abroad, you'll come across "opa!" frequently.
Opa as a Sound of Acclaim
The use of "opa!" as a sound of acclaim we've heard from Greeks as well, but this seems to be a case of a Greek word wandering off into a brand new meaning, and then returning back to the language, at least among the workers in the tourism sector.
It is used now as a call for attention, an invitation to join in a circle dance, or a cry as the flame is lit on the saganaki—a melted cheese dish that is traditionally flambéd at the table by the waiter.
The Real Meaning
The actual meaning of "opa!" is more like "Oops" or "Whoops!" Among Greeks, you might hear it after someone bumps into something or drops or breaks an object. Because of this, you may also hear it during the now rare breaking of plates in Greek restaurants and nightclubs as a sound of praise for the singers, dancers, or other performers. This may actually be where it got its extra meaning as a sound of praise—originally used after the breakage occurred, and then became associated with the act of praising the performers.
Other Uses in Popular Culture
"Opa!" is also the title of a song by Giorgos Alkaios which was submitted as the official entry for Greece in the international song contest Eurovision for 2010.
However, oops, it did not win. It alternates with the word "Hey!" in the song, which works as a translation of Opa, too.
Not Just a Word, A Lifestyle
Greek-American columnist George Pattakos takes opa! even farther—presenting it as a lifestyle lesson and possibly even a new entry into the annals of Greek philosophy.
In a piece for the Huffington Post, owned by very Greek and opa-lifestyle-embracing Arianna Huffington, he describes what "opa!" means to him and how adherence to his principles of opa! can enhance or change your life. He's even founded a center based on his principles of applying opa to everyday life, dedicated to the practice of "The Opa! Way" and manifesting your inner Greekness, which he says you can have without being actually Greek.
In a way, the word opa has undergone the same type of transformation as that of the name "Zorba." Nikos Kazantzakis' character and the movie that was made from his book have become synonymous with a love of life and the triumph of the human spirit yet both the original book and the movie surprise modern readers and viewers with the darkness of many of the episodes depicted. Yet to hear the word "Zorba" we just think of the expression of joy and triumph over sadness just as opa! has come to mean something similarly bright and positive.