Which version is “right”? Any one of the three is acceptable. However, whichever one you choose, be consistent by using only that spelling of the word throughout the manuscript. 

Personally, I prefer O.K. over OK and okay. OK also is an abbreviation for Oklahoma, so there is the potential for confusion in using that spelling. Okay takes up more space, and my prejudice is against longer versions of a word when shorter versions will do.

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My name is Rob Bignell. I’m an affordable, professional editor who runs Inventing Reality Editing Service, which meets the manuscript needs of writers both new and published. I also offer a variety of self-publishing services. During the past decade, I’ve helped more than 300 novelists and nonfiction authors obtain their publishing dreams at reasonable prices. I’m also the author of the 7 Minutes a Day… writing guidebooks, four nonfiction hiking guidebook series, and the literary novel Windmill. Several of my short stories in the literary and science fiction genres also have been published.