I always thought that first variant (both letters written uppercase) is appropriate (maybe because I'm a Windows user), but recently I see more usages of second one (only first letter written uppercase). Which one is more eyes- and mind-friendly?
Stick with is most common and time-proven. OK (with caps) has been used always. You can see its wikipedia page, is written using both caps.
If you have space in your UI, maybe consider using a more indicative label like:
'Delete my stuff' 'Sign me up' 'Show me the report'
or the name of the page it takes you to.
It's also worth thinking about how the appearance would affect someone reading aloud. OK is recognisable and forces a break between the letters ("oh kay"), while I've seen people slow down as they remember that Ok is the same thing and isn't "ok" or "och".
Whether you use OK or Ok could also be depending on the country/area - in Denmark, e.g. in informal texutual conversations the all-lowercase ok is seen a lot.
That being said, I would go for a button-label which reassures the user of the action which he is performing - as kelbyuk suggests.
Either 'Ok' or 'OK' should be fine, as long as whatever action about to be triggered by the button is crystal clear to the user.
That being said, to me, 'Ok' is cushy whereas 'OK' is serious, so I would use them accordingly based on characteristics of the product.