Do not use OK in academic writing. It is too informal. (It may be true as Burchfield says in his edition of The New Fowler's, that this is "possibly the only English word universally recognized by foreigners throughout the world" - but this in itself might make academics suspicious of its use. "If it is so easy to use, and so simple a word, a student who uses it must be simple, and not good enough to pass" is roughly how the pedant might instinctively react.)
If you must use it, bear the following advice from the Guardian in mind: "OK is OK; 'okay' is not." In other words, write the word as a simple pair of letters, and do not spell out how it sounds. It is also permissible to write it with two full stops: O.K.
And if you use it in academic English, only use it as part of a quotation. In that form - and only in that form - you may use this interjection as a verb, spelled out: 'The General okay'd the Lieutenant's plan'.
You may be interested to know that the expression 'OK', whose first recorded use is in the 19th century, is probably an abbreviation of 'orl korrect', a humorous variant of 'all correct'.