What it means
From Latin oculus uterque. ISMP lists OU, OD (right eye) and OS (left eye) as error-prone, because they are mixed up with each other and with AU, AD and AS, the abbreviations for the ears. Eye drops have been put in ears, and ear drops in eyes, because of this.
For the nurse
If an order uses OU, clarify it before giving the drug, and write "each eye" in your own notes.