MartinX
I'm not sure if that's OLED itself and more to do with android 6+ which those phones come with as their lowest OS. (though I believe you can get Lollipop on the s6? Mine came with Marshmallow and I sent it back before looking into "downgrading" the OS)
I can use any OLED phone on Lollipop or below but the moment I use Marshmallow or above it's immediately uncomfortable. I can also use OLED windows phones. All with zero issue for as long as I want
Tested this across a few different OLED phones and the result was the same every time I updated to Marshmallow I would be met with eye strain and headaches. Now there's also the chance that new OLED displays may also be an issue now too but you can't really separate phone and the OS now. Installing Lollipop on an LG V30 for example is not going to happen. There's also what's driving the display may have the same issue as with newer Nvidia cards do where no matter what there's hardware based strain issues.
I think this is also a problem with a lot of smart TVs now for people like us too. Maybe the screens okay but you'll never know because the OS it's using is causing strain so it has the same outcome anyway.
I assume some people just can't use OLED comfortably though.
The S6 had some of the most worst PWM I've ever seen though which is probably not helping at all. I can't believe Samsung find that acceptable.
Edit: this was all basically said before by KM SILLY ME