I own both an Oasis (current version) and Kobo Sage. I suffer from frequent migraines. E-ink has always been better than a backlit screen but now I am noticing that the Oasis is a bit easier on my eyes than the Sage. It’s especially notable when I’m tired or in pre-migraine.

Any idea why one would be easier on me than the other?

I use Bookerly on both. Oddly, I can tolerate a smaller font size on the Oasis. Light levels are comparable. The Oasis screen seems a bit greyer than the Kobo screen, which is whiter. Warm light is set to automatic on both.

Any other comparisons which might be helpful?

I prefer the Kobo software and Sage screen size but I’m definitely finding myself gravitating to the Oasis more and more for comfort in reading.

Any ideas welcome!
I've had an Oasis (gen2 recently) and do have a Sage.

It has to absolutely be only the front lights. The actual eink can't be the cause, assuming the font & size is the same and it's not a reading glasses issue. I started getting terrific stabbing & sickening headaches in the late 1990s and it was simply I needed reading glasses. I'd never used them and my laptop screen was closer than my previous CRT! First laptop. Laptops are handy, but not always ergonomic, so years later I used a separate screen and keyboard on the laptop at home.

I'd recommend never using front light and only using suitable for you ambient lighting.

Migraines are not well understood.

There also can be very severe headaches that can be photosensitive from either badly congested sinuses, alcohol, dehydration, lack of blood sugar and wrong (or none) reading glasses, or stress. Or more ominous causes. Many headaches can also be accompanied by nausea and not respond to traditional tablets.

If dark chocolate (75% to 85% cocoa) makes it worse it may be be classic migraine. If that relieves the symptoms then it's sinus related. Decongestants might give short relief, but experts saye they make sinuses worse. Cocoa is safer than cortisone type drugs, possibly it's the Theobromine that helps.

Migraine can be triggered by chocolate or oranges (Don't try Terry's Chocolat Orange!) and perhaps it's the caffeine (so tea, coffee, cola bad too) or citric acid (any citrus food and added to other things). However the underlying causes of migraines are unknown.

Make sure it's not something else, because most of the "not Migraine" migraine-like headaches are more treatable and better understood.
It does seem logical that it must be the front light. I actually hadn’t thought of that.

They are migraines. I’ve been diagnosed and treating them for years. Sadly, the treatments only work some of the time.

Thank you!
In that case use a nice desk / reading lamp (LED yellow stick type or halogen/tungsten are best, not CFL or any kind of compact LED) that's OK for paperbacks and use that with the front lights off.

Happy reading!
About 5 years ago, I was also a migraine sufferer, so I know your discomfort.

Mine was from the mercury amalgam fillings in my teeth, they were leaking. I just happened to find a website one day talking about this. I took the plunge and thought what do I have to loose, had them removed and refilled with the new white stuff they put in. Never had a migraine since.

Since then, I can sit longer in front of devices, computers, etc. I still get headaches though, but that is because of my health, going to work on that this year.
you can check with an RF meter, if your device emits radiofrequency in high amounts;
i always turn off wifi/bluetooth, when not strictly needed (almost always), but some devices don§t seem to care and still give some unhealthy dose of radiation out it seems.
Totally bogus.
Been disproved 100%

No one can sense RF unless the power is so high as to heat your skin. No Wifi or BT device can do that.

I always have WiFI & BT off to save the battery.
There was a test done due to the new smart meters the electrical utility was installing since oddly for a device that connects once per month, there were people who were complaining about all kinds of ill effects from the radiation. They had several of the people who complained about being able to sense the RF radiation from the meters and had them sit in a room with a meter with a switch attached to it allowing the wireless communication module to be turned on or off.

When the switch was in the on position, there were complaints of headaches, nausea, etc. from the test subjects. When the switch was in the off position, the symptoms cleared up. A somewhat odd result from using a dummy switch and an electrical meter without the wireless communication module installed.

And BTW, the RF radiation from a WiFi or Bluetooth module installed in your ereader, etc., is non-ionizing RF radiation at a much lower level than the safety limits for non-ionizing radiation not to mention effects of the inverse square law. As anyone who has browsed the FCC or other testing agency listing rapidly realizes, the majority of the test are concerned with the amount of RF emitted by the device.

Do you worry about living near a coal fired power plant? The average radiation level from a coal fired plant is about 1.9 millirem per year which is a level that would cause a nuclear power plant to be shut down. By comparison, the level downwind from Three Mile Island was about 1 millirem per year after reactor #2 did a partial meltdown. In comparison, the average background radiation level is 100-400 millirem per year.

You might want to check the following 2 items and move on from there, The Electromagnetic Spectrum: Non-Ionizing Radiation and The Electromagnetic Spectrum: Ionizing Radiation
Interesting reading. Thanks.
I’d be more worried about the dangers of a mobile phone clamped to my head a lot of the time like a lot of youngsters seem to do these days, than radiation from computers and screens.
Sorry slightly
Who wouldn't? The pollution they put out is disgusting, and radiation is the least of it.