What does OS mean in musical terminology?

In musical terminology, "O.S." stands for "Ottava sopra", which is an Italian musical term meaning "an octave higher." It is used to indicate that a passage or section of music should be played an octave higher than written. O.S. can be used either as a global instruction, applying to a continuous section of music, or as a localized one, indicating a temporary elevation in pitch. It is most frequently employed in vocal or instrumental arrangements to adapt the pitches to a specific performer's range or to achieve a particular musical effect. The opposite of O.S. is "O.B." or "Ottava bassa," which signifies that a passage should be played one octave lower than written.