It is a curious fact that a substantial percentage of two-letter words are interjections; of the 227 words in the "Dictionary of Two-Letter Words" issued by the Biliteral Society and reviewed below, grouping interjections according to feelings, commands and statements, show the great variety of messages that can be sent.
Pollack, Philip L.
Word Ways: Vol. 18
, Article 11.
Available at: http://digitalcommons.butler.edu/wordways/vol18/iss2/11