For American usage, the "bible" today is
Garner's Modern American Usage. Professional editors rely on its guidance and Bryan Garner is affiliated with many of the leading language resources, including the
Chicago Manual of Style.
Garner's Modern American Usage (3rd ed. 2009) states (p. 588):
Quote:
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okay; OK; O.K. Each of these is okay. Although OK predominates in highly informal contexts, okay has an advantage in edited English: it more easily lends itself to cognate forms such as okays, okayer, okaying, and okayed.
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Bottom line is either is acceptable although one will predominate based on context.