I generally spell it “okey” in very casual settings. When I was a kid, I read a book where the author spelled it “okey.” I just loved it, and have been using it since.
And, as further evidence of my dorkitude, I often say it more like “ookie,” too.
I have no excuse for that one.
Don’t forget that thanks to South Park we now have m-kay or mk to get around the censors in polite company. So m-kay being a four letter word still gets it’s meaning across, when a f and three underlines doesn’t print correct like this f___ or a bunch of random symbols isn’t your thing. I guess with the random symbols, _uck is the one word in our language that has the most accepted ways to represent it and still be identified.
I like Daffy, but my husband prefers Donald.
m-kay 
In Japanese it would be <オーケイ> – i.e., “ookei” romanised bacxk again – unless they used romaji “OK”.