Scott: Assuming you are ONLY using the Oppo 105 as a DAC and your MAC Book Air is the source for all music, I do not believe you need a monitor for the Oppo 105. When my MAC Book Pro was connected to the Oppo 105, used as a DAC ONLY, I did not use a monitor for my Oppo 105. As I stated above, once you set the Oppo 105 input to USB, you are ready to start playing music from your MAC Book Pro. As someone stated above, there might be some audio setting needed for fine tuning, but, most likely, these settings are not required for your application. When I used the DAC only in my Oppo 105, I used the standard Oppo settings and made no changes. Once the MAC is connected to the Oppo 105 via the USB cable, all audio controls are done on the MAC using iTunes and the volume control on your amplifier.
If you want to review the settings in the Oppo 105, you can temporarily connect it to your TV (or any monitor) and make the required changes and return the Oppo 105 to your audio system.
If you plan on using the Oppo 105 for video or for playing CD's from the Oppo (and not the MAC Book Air), an external monitor is definitely required. You need to verify this but I believe Oppo Digital changed the operation on the Oppo 105 so that an external monitor is not required for playing CD's.