I had the same issue with UA before. I called OZ and they told me that the OZ system doesn't have a field for Middle Name.

For an existing reservation on UA, I asked the agent to move my middle name into the field for the First Name and that allowed the OZ membership number to be added. For new reservations, I put my first and middle names into the First Name field (with a space) and it takes the OZ number without any issues. HTH.