Which abbreviation would best describe conditions affecting both eyes?

The abbreviation that best describes conditions affecting both eyes is "OU," which stands for "oculi uterque," a Latin term meaning "both eyes." In medical terminology, this abbreviation is specifically used to refer to conditions, treatments, or observations that pertain to both eyes collectively, making it the most suitable choice for the question.

In contrast, "OD" stands for "oculus dexter," which refers to the right eye, while "OS" stands for "oculus sinister," referring to the left eye. These two abbreviations specifically describe conditions affecting only one eye at a time. "NPO," which means "nil per os" or "nothing by mouth," is unrelated to ocular conditions and refers to withholding food or fluids from a patient, typically before surgery or certain medical tests.