The op full form is Opus number. The opus number is a “work number” assigned to a musical composition, or a series of compositions, to denote the composer’s historical order of production.
Music publishers used to issue opus numbers to compositions or groups of compositions to show the chronological sequence of a composer’s works until the 1800s. So, imagine a perfect world in which you come upon Van de Garre’s work. There’s a carefully categorized catalog of the composer’s works, from op. 1 through op. 100, their final work. It’s a wonderful record of their evolving interests and styles; one may listen to the composer’s progress from start to finish.
But things aren’t that straightforward. Because opus numbers were frequently assigned by music publishers rather than the composers themselves, things get a little wild. Publishers would frequently publish a series of compositions under a single number during the classical era. Haydn’s Op. 1 has six different string quartets, for example. Additionally, if a composer sent a piece of music to two distinct publishers, it’s possible that it obtained two different opus numbers. Things get much more complicated when you consider that vocal music rarely receives an opus number because a title like “An die Musik” is easier to recall than Piano Trio No. 3 in F. Since opus numbers were mostly a fixture of the publishing sector, works written for the stage, such as operas, lacked the numbered insignia as well.
Composers began assigning their own opus numbers before sending their works to publishers after Beethoven. However, it was still far from ideal.