Douglas Crockford goes into why he chose those 4 letters in this page: http://www.crockford.com/wrmg/base32.html http://www.crockford.com/wrmg/base32.html. I,

Douglas Crockford goes into why he chose those 4 letters in this page: http://www.crockford.com/wrmg/base32.html http://www.crockford.com/wrmg/base32.html. I, L, and O can be confused with the digits 1, 1, and 0, respectively. He omits U to avoid "accidental obscenity."