The Wordfind website lists a total of 101 two-letter scrabble words. The highest scoring of these words are ex, ox, jo, ka and ki, along with xi, xu and za, though the purpose of two-letter words in scrabble is clearly not for their point values but for their strategic value.
The most important use for two-letter words is so that tiles can be placed parallel to other pre-existing words, allowing for more options in word choice and higher point combos in creating multiple words in a turn. The huge increase of flexibility provided by a vocabulary of two-letter words allows for a Scrabble player to play at a higher skill level.
Two-letter words also allow a player to discard unwanted tiles during the later portions of the game when the board is more cluttered without needing to pass a turn. Finally, blocked off portions of the Scrabble board can by accessed through the effective use of two letter words. While most of the legal two-letter words in Scrabble are fairly obscure English words, some, such as ox, up, he and hi are common and used in spoken language as well. Wordfind has a comprehensive list of two-letter Scrabble words on its website.