The following are cropped screenshots from the sample videos posted on GSM Arena of the OnePlus One, along with one from their iPhone 6 Plus review. The 4K YouTube video is 19Mbps, so needs a fast cable or fibre-based connection to play in 4K, unless downloaded (I used 4K Download.)
Order: OnePlus One in 1080p, 4K (played downscaled to 1080p) and iPhone 6 Plus in 1080p:
The only thing missing is optical image stabilisation.
The Nexus 6 also does 4K video and has optical image stabilisation.
While I don't plan upgrading to a 4K display or TV anytime soon even if I get the phone, the additional resolution will provide better cropping ability when downscaling to 1080p.