On the back of the OWL Modular, on the main board, there is a 3-pin header which selects boot mode, and another 2-pin header. If you connect a jumper across the 2-pin header you will be able to power the digital board over USB.
However, bad news: this doesn’t power the analogue circuitry, which runs from the +/-12v supply. This means that the audio inputs and outputs, CV and ADC won’t work. You can talk to the OWL over USB MIDI, but you won’t get sound out of it and the parameters will be stuck in the max position.
What you need is really +12v and -12v going into the OWL.