I think okay is used in narrative prose and sentences e.g "Father okayed admission of his son to local sports club". Whereas O.K. is more often used in conversational sense E.g a boss says: "you will go to the warehouse tomorrow and purchase these items." The employee says: "O.K. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.