Back in the 80/90s I thought about buying the 20 vol edition of the OED offered at a special mark down price ... I didn't...

Wiki says
The OED's official policy is to attempt to record a word's most-known usages and variants in all varieties of English past and present, worldwide. Per the 1933 "Preface":

    The aim of this Dictionary is to present in alphabetical series the words that have formed the English vocabulary from the time of the earliest records [ca. AD740] down to the present day, with all the relevant facts concerning their form, sense-history, pronunciation, and etymology. It embraces not only the standard language of literature and conversation, whether current at the moment, or obsolete, or archaic, but also the main technical vocabulary, and a large measure of dialectal usage and slang.

It continues:

    Hence we exclude all words that had become obsolete by 1150 [the end of the Old English era] ... Dialectal words and forms which occur since 1500 are not admitted, except when they continue the history of the word or sense once in general use, illustrate the history of a word, or have themselves a certain literary currency.