So the present owner of the corral, Robert Love, tells me. Good enough is a phrase used in psychotherapy jargon, and in the 1970's the word OK found its way into that branch as well. I was OK, and you were OK too. In the text database British National Corpus 5 it is at place number 947, according to a count made in 1996. Still, text collections like that do not yield information about the use of the word OK as a cue marker, as a general affirmative in speech, or as an execution initiator in computer programs.
Normal language is, of course, a very diffuse notion.
It is not possible to create a generalized list of the most common words, only a list according to a certain corpus of texts. The representativeness depends upon how and when the material in the corpus has been collected. There is also a problem how to deal with different forms of the same word. If we count each form as a word in its own right, the above list is correct. But if we regard be, are, is, am etc. as one word, this lemma would immediately jump up to second place after the definite article.