@alanna82: For a bit of background about what makes a word acceptable or unacceptable in Scrabble, see this article - http://www.word-buff.com/scrabble-word.html - it also contains a word checker which will agree that ab and ad are both acceptable and OK isn't.
OK is actually an acronym, hence the double capital letters. It stands for Oll Korrect, an intentional misspelling of All Correct. It caught on amongst 19th Century society in a similar way to how speaking with homophonic numbers has caught on in "txt spk" today (UR 2 L8 M8! ROTFL etc....) Anyway, as the article suggests, any capitalised words, be they proper nouns, acronyms or other, are not acceptable, meaning OK is not okay.
I think the reason words such as ab and ad are acceptable is that they have simply become words in their own right, listed in dictionaries synonymously with their longer counterparts. Weirdly, some other acronyms, which many people don't tend to realise are acronyms have become legal - words such as LASER (Light Amplification by Stimulated Emission of Radiation) and SCUBA (Self Contained Underwater Breathing Apparatus) are generally not spelled with capitals and have just been absorbed into the English language as single words. I'm not sure if it is more strange how quickly dictionaries have adopted these newer, longer acronyms as bona fide words, or that they have failed to do so in the case of OK. Regardless, these are the rules we've been given. Someone has to draw a line somewhere and it has been drawn by lexicographers far more qualified than myself. Without official lists such as SOWPODS and TWL, anyone could just invent words from whatever tiles they held in word game tournaments, which would reduce the whole game to a pointless charade.
The OED faq gives a brief insight into how lexicographers go about the business of selecting words for inclusion in their dictionary if you're interested - http://www.oed.com/public/faqs/frequently-asked-questions/ |
The Keyboard Alphabet 
Every 4 Letter Word
Every 5 Letter Word
