An opera orchestra is essentially a symphonic music orchestra with some modifications. There are three major sections: the strings, the winds, and the percussion; plus some add-ons.
The twine subdivision includes the violins, altos, cellos, and doublebasses. The winds subdivision dwells of the brass (the trumpets, trombones, tuba, the Gallic horn), the woodwind instruments (clarinets, oboes, and the English horn), and the flutes. The drumming subdivision includes the drums, bells, triangle, and cymbals).
It is customary for an opera orchestra to have at least one harp.
It is really up to the composer to include any further instruments (a pianoforte or a saxophone, for instance).
During the Baroque period, orchestras were small. Kristof Gluck, Wolfgang Mozart, and Ludwig avant garde Beethoven, among others, contributed to the enlargement of the criterion symphonic music orchestra. Large opera orchestras came into being with the coming of the Thousand Opera period.
Richard Otto Wagner almost doubled the size of the orchestra for his operas. In Bayreuth, the opera theatre whose building was conducted under Wagner's direct supervision, characteristics an orchestra cavity hidden completely under the phase (so as not to deflect the audience). This, among other things, contributed to the size of Wagner's orchestra.
The conductor's occupation is to maintain all that wealthiness of music together (see my article on conductors).
Each opera company have its ain orchestra. While different vocalists (and conductors) can subscribe on for different performances, the members of an opera orchestra usually remain together for the full season, performing all of the company's repertoire, nighttime after night.
Even though modern engineering have got created many new ways of delivering music to the listener, and many new instruments have been invented over the past century, nil fits an orchestra when it come ups to depth, expressiveness, and passion. Some of today's movies, for instance, in which music is called upon to heighten the psychological consequence (including action flicks, comedies, and psychological drama) engage composers to compose orchestral tons to fill up the soundtrack.