All three phones record video up to 2160p/30fps - not that you'd expect otherwise. There are the obligatory 1080p/30fps and 1080p/60fps modes as well, as well as slo-mo and timelapse options.
2160p footage gets a bit rate around 42Mbps on the OP5, 46Mbps on the iPhone 7 Plus, and around 48Mbps on the Galaxy S8. The Samsung phone records stereo audio with a generous bit rate of 256kbps, the OP5 only spares 96kbps, but still in stereo, while the iPhone records a miserable mono track, though that gets a 82kbps bit rate all on its own.
When properly supported on a tripod, the three phone capture excellent 4K videos, albeit with somewhat different vibe. The Galaxy S8's footage has the most vivid colors and highest contrast, and looks the sharpest, though a little artificial. The OnePlus 5 takes the middle ground in punch and contrast, and looks more natural, while also capturing a smidgeon more detail than the iPhone.
In 1080p/30fps, OnePlus' stabilization kicks in, heavily cropping and narrowing the FOV. The footage is soft and detail is underwhelming. The iPhone 7 Plus does a better job, and so does the S8, though the S8 still looks overprocessed.
Oddly enough, the OnePlus 5's 1080p/60fps footage is much better. We gather that not having to do all that stabilization calculations and just recording what the sensor captures is what does it. The Galaxy S8 and the iPhone 7 Plus handle the high frame rate without much fuss, and their 60fps footage is similar to 30fps, only smoother.
The OnePlus 5 and iPhone 7 Plus have another fight to settle - their telephoto cams can record video as well. In 4K the iPhone 7 Plus is slightly superior, capturing more detail and having better colors - the OP5's footage is somewhat muted in both colors and contrast, though it isn't half bad.In 1080p, there's really no contest - the iPhone 7 Plus' videos are superb, be it 30fps or 60fps. The OnePlus 5, on the other hand, captures half decent footage in 30fps, and downright appalling 1080p/60fps videos with its telephoto cam
The iPhone 7 Plus and the Galaxy S8 support electronic stabilization across all these modes, while the OnePlus 5 can only stabilize 1080p/30fps footage. That's a rather unfortunate limitation, made even worse by the fact that the OP5 doesn't have OIS in the first place - both others do. Effectively, you're on your own, and it shows.
Shooting a 4K video on the OnePlus 5 while walking is best avoided - with no stabilization at all it stands no chance against the OIS+EIS combo of the iPhone 7 Plus and Galaxy S8.
It's entirely different in 1080p/fps, where the OP5 does offer stabilization. You can't turn it off though, but then neither can you on the iPhone - only the Galaxy S8 gives you that option. One beef we have with the OP5's implementation is that you don't get an accurate viewfinder representation of what's being captured - it shows you the view before the stabilization algorithm crops out the edges for smoothing out shake.
The OP5's EIS is easily as good as the iPhone's and S8's in producing steady footage both while walking, or standing in the same spot just holding the camera. Where it fails is panning - it takes a split second longer to determine what you're doing, and when it does, it catches up abruptly. It's not a huge deal, but it's noticeable.
Video winner: Apple iPhone 7 Plus. The iPhone captures detailed videos with both cameras in all modes with pleasing colors and a natural look. The competent stabilization is also a huge plus. The Galaxy S8 is a close second, mostly due to a more overprocessed look - if you prefer that, it could be your winner. The OnePlus 5 lacks stabilization in all but 1080p/30fps, and while stable, those look bad. 1080/60fps videos from the tele camera are dismal. 4K is great from both cams, but you need a tripod.