I need it because is an EMEGENCY
It is an event item. you can use it to get shaymin if I remember correctly, but the event is already over, so you can't get it. :(
http://www.serebii.net/itemdex/oaksletter.shtml
Oaks Letter was a speical event to catch shaymin. But it was only available from Sept 28 --- Nov 8 2009
Sorry the event is over!!
I think the more interesting question is how did you not see the duplicate question ;) It would have come up when you asked yours: http://pokemondb.net/pokebase/234/where-you-get-oaks-letter-in-pokemon-diamond