OBS hates multichannel interfaces!
It was originally made for (and never changed from) the stereotypical bedroom streamer, that has a surround-sound game and a mono mic, and that's it. So no channel selection, only device, and each entire device is assumed to be a single surround source with a format determined solely by the channel count. OBS downmixes that to stereo or whatever it's set for in
Settings -> Audio, and THEN gives you that mess to try and make something of. Not good.
If OBS's setting matches the interface's channel count, then it's all good with no mixing...so long as you understand the assumed labels for each of those channels and adjust your workflow accordingly. (following a set of 8 conference mics on an 8-channel interface all the way through to an actual 7.1 rig, will result in each mic going exclusively to a different speaker, one of which is the subwoofer...)
If you want separate control of each channel, then you can't use OBS for that. You have to use an external tool, like a DAW or physical console, to produce a single stereo signal for OBS to pass through completely unchanged. OBS, then, is completely silent except for that one stereo source from the external tool.
Did your consider using ASIO ?
...
That only works on Windows, because Windoze's audio is so stupid that it has to be bypassed for serious work. That's what ASIO does, and why it exists. Because only Windows has that problem, only Windows has that solution.
The ASIO plugin does have a channel selector, which would solve this problem too, but once you've become fluent in a *good* external audio tool, you really don't want to use OBS for audio anyway. OBS sounds okay - no problems there - and its filters follow the industry standards for how each one should work, but the complete lack of metering or any other indication of what's going on makes them the hardest of anything to use.