OK, O.K. and okay are all correct. OK and O.K. are always supposed to be capitalized in writing as the abbreviation for okay, but often they are not.
Okey is not a stand alone word but it is part of the terms "okey dokey and okey doke" which are informal versions of okay. We use them not so much as slang but because the two words rhyme and sound nice to an English speaker's ears.
Okieee is not a word. If you saw it in writing, the author was trying to communicate the way in which someone was stretching out the word "OK" in order to convey emotion through the word.