Low light is a close fight between the OnePlus 7 and the Redmi K20 Pro, especially as the two extra cameras on the latter aren't of much help in these conditions.
When shooting with the main camera, the OnePlus has better detail preservation and also generally better white balance. The details on the Redmi K20 Pro look more smudged and the white balance under tungsten lighting is too warm. The Redmi K20 Pro images are also oddly lacking in contrast, which gives them a hazy look, especially visible in the dark sky. The only area where the Redmi K20 Pro does better is in dynamic range as it is able to better preserve the highlights in bright areas like signboards.
Both cameras also have a night mode option and here once again the OnePlus 7 pulls out on top with a better implementation of the feature. There were very few scenarios where the Redmi K20 Pro night mode produced a better result but that's only because the image was brighter than what the OnePlus 7's Nightscape feature managed. However, every single time the OnePlus 7 image had better detail, dynamic range and overall quality. Of course, neither one comes close to matching the night mode on Pixel 3, Huawei P30 Pro or even the Galaxy Note10+.
Despite having the same sensor and optics, the OnePlus 7 is able to beat the Redmi K20 Pro in low light because it has OIS, which allows it to shoot at a much lower ISO and longer shutter speeds. The Redmi K20 Pro, on the other hand, was shooting at five digit ISO most of the time with shorter shutter speeds, just to get comparable shots.
With the 2x zoom, the two cameras are basically identical as even the Redmi K20 Pro is now reduced to digitally zooming in as it won't use its dedicated telephoto camera in low light. The Redmi K20 Pro was once again using higher ISOs and shutter shutter speeds but not by much. This meant that the only difference between the two phones was largely the colors and the contrast and the OnePlus 7 did better here overall.
The Redmi K20 Pro can, however, use night mode even with 2x zoom. It is still not using its dedicated telephoto lens but you can actually get slightly better results with it.
As for the ultra-wide-angle, the image quality wasn't very good at all unless there was sufficient light, so it's not really usable, especially since you can't even use the night mode with this lens.
Winner: OnePlus 7. This is a classic example of how much of a difference OIS can make on its own when everything else is equal. A fairly decisive win for the OnePlus 7 here as the non-stabilized Redmi K20 Pro is left to bump up its ISO to scary levels just to produce a usable image.