Wide angle large format lenses are the easiest fit for the Oxygen, they require virtually no effort to adapt, and will work well. They are on the other hand, rather slow (usually f/5.6 and up).
Bronica ETR (6x4.5) are another great match for the Oxygen, and it's what I use on mine. They do require that you remount them into a mechanical shutter though, unless you can build an electronic contraption to make them work in their electronic seiko shutter. This disassembly is, if done right, non destructive. This means you will be able to remount it into it's original shutter and have a working Bronica lens at the end of the day, if you so wish. Also, you can buy lenses that are already in non function shutters, giving them new life, and at the same time saving yourself some money.
The Process is as follows , remove the beauty ring, remove front optical cell with spanner wrench (it's screwed in with locktite, so it will sit quite hard). remove back optical cell with spanner wrench. Remount into mechanical shutter. Done. Here is a link to a complete comprehensive disassembly, if you want to do that, although as mentioned above, that is unnecessary for this project.
Shutter compatibility : ETR Lenses from 50mm to 250mm will fit into copal or compur size 0 shutters. SQ-A lenses should also fit into these size 0 shutters.
Protective lens cone for Bronica lenses :
Some Bronica Lenses have bulbous front elements, to protect them I have made cones that fit in between the front optical block and the shutter. these require three M2 set screws, and require you to thread your 3D Printed parts with a M2 tap
Focusing scale :
50mm focusing scale for 25-55mm chinese helicoid (print on A4 paper)
You can mount PC-Nikkor lenses on to Copal 3 Shutters to make them work on this camera, your max shutter speed will be 1/125th of second but if you can live with that you might get some great results. Cameraventures has a few in stock.