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.
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".