Unfortunately, it cannot do MIDI over USB. There are two ways to upgrade the firmware on the OT per the manual. The first way is through USB, where you drop a firmware update onto the CF card. The second way is through MIDI, where you send sysex over the actual MIDI ports, not through USB.
I want OB for my OT too, but with the current hardware it is just not possible. The USB connection is so basic that the OT and your computer can’t access the CF card at the same time, which is why when you go into USB mode the OT is basically paused and nonfunctional.