You mean this? (1st google hit):
Discover the key features of the Pioneer DJ DJM-900NXS, 4-channel club digital mixer (black)
www.pioneerdj.com
Just going by the labels in the picture, it appears to have 8 channels on USB, arranged as 4 stereo pairs. OBS hates that!
OBS was made for the stereotypical bedroom gamer/streamer, who has a mono mic as one device and a surround-sound game as the other. Not much beyond that. So, OBS selects a device, and that's it. No channel selection.
Multichannel is assumed, with no way to change it, to be a single unified surround source. So it downmixes that according to the standard channel assignments (front/back corners and sides are 3 stereo pairs that get mixed into a single stereo pair at different levels, center goes to both sides, subwoofer disappears), and THEN gives you that mess (if it's actually anything other than real surround) to try and make something of. No way to change that.
ASIO *might* help, as I've seen one (and only one) screenshot of a channel selection in OBS, on a rig that had a working ASIO driver, but considering where OBS came from and the pile of band-aids that its audio system already has on top of that, I wouldn't count on that in general.
If you must use that interface, use something else to receive it and translate to something that OBS likes. Pretty much every DAW (Digital Audio Workstation) can do that, *and* it allows you to completely bypass OBS's audio mess entirely. Do all of your audio work in the DAW, not OBS, so that OBS has exactly one audio source at all, which is the final finished soundtrack to pass through completely unchanged.
Or, you might use multiple separate stereo-only sound cards, each of which really is stereo-ONLY.