hey if you have the oxygen pro in preferences goto midi devives select oxygen pro on the midi input right click and enable it then enable control signals
these settings allow midi signal to be sent from the midi keyboard to the computer
next goto OSC settings and enable midiIN3 this setting will allow the midi keyboards play stop record and loop buttons to work on the face of your midi keyboard
now you have to enable your keyboard built in settings so first hold the daw select button make sure its on reaper
then hold the preset select button and scroll over to a user preset
this will unlock the faders and knobs for cc midi assignment
if you arent on a user preset then the faders and knobs will be preconfigured and locked from user reconfiguration
the last thing i did to be able to map any of my oxygen 61 pro's controls to virtual daw parameters such as track volume and pan etc.. to do this I used the REALEARN plugin.
just download and install realearn into your plugins
follow these instructions this tutorial will help but you need to skip to timestampm 4:29 for the realearn tutorial
https://www.youtube.com/watch?v=UrYrAxnB19I
you can find other youtube tutorials that show you how to do what i just described but you will have to do the basic search to find oxygen 61 pro and how to get connected and setup
however some of the info in this post I figured out on my own and could not find else where
good luck