I recently found out a solution to get rid of the “USB midi host” device need when connecting a keyboard to the OP-1 (G).
I was usin a Hobbytronic device, working well but… requiring power adapter, a set of cables, sapce consumming (having to carry 1 more device and find room in the box…).
I found a solution which is, to me, handier:
an android app ( I found that midi keyboard from Dreamhound studios LLC (on google app store) - I have no relation to them), an… android smartphone (OTG compatible) → an OTG usb cable → a littel USB hub (1 to 2) an usb cable to the keyboard and 1 to the OP-1.
Works great, enjoy!