When the abbreviation op. is followed by a number it stands for this

Below you can find the When the abbreviation op. is followed by a number it stands for this Jeopardy Words Answer. The answer to When the abbreviation op. is followed by a number it stands for this has 4 letters. This clue belongs to the Music category and is part of Jeopardy Word Level 550. If you scroll near the bottom you will find other clues and answers part of level 550