The abbreviation of the colloquial expression ohkay seems not to be o.k., but one of:
Maybe it is in itself an abbreviation of words that are only used in abbreviated form. Which is the correct one?
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This Wikipedia article says "Whether this word is printed as OK, Ok, ok, okay, or O.K. is a matter normally resolved in the style manual for the publication involved. Dictionaries and style guides such as The Chicago Manual of Style and The New York Times Manual of Style and Usage provide no consensus."
Older sources do often write "o.k." or "O.K.", but the periods seem to be less common in the last few decades.