I think the upcoming Kordbot will be able to do it, but is there any other MIDI controller / controller keyboard that has USB host functionality? i.e. can plug the OP1 in without a go-between?
kilpatrick sequencer has usb host feature
Also check out http://www.juno.co.uk/products/mode-machines-cerebel-usb-to-midi-serial/564328-01/
If you couple it with a 5v usb battery it makes a nice portable solution to usb>midi
Upcoming Parva synth does usb host as well