It seems you're looking for a musical term or note value that can complete the statement "O - ____ =". Given the options of a quarter note, whole note, half note, and quarter rest, I'm interpreting that you want to fill in the blank with something that relates to a whole note, represented by "O."
In music notation, "O" usually represents a whole note, which looks like a black hollow oval. It seems you might ask for a subtraction of note values.
In this case:
- The whole note (O) minus a half note would leave you with a quarter note.
So, the answer could be:
O - (H) = Q (Where O = whole note, H = half note, Q = quarter note)
Could you clarify if this fits what you need or if you are looking for something else?