Please be warned about this library :-
After looking at the code it is clear that the Arduino is not using Oauth or tweeting the message. All it does is send the message to NeoCat's website where some kind of script then tweets the message.
Also, you have to sign up on his website to use it, meaning he then has your Twitter ID and Password.
I am aware that this may be a solution to the Oauth protocol and one of the ways that it works. But users need to be made aware that the Arduino is not doing the tweeting, the Arduino is simply sending your Oauth token and your tweet to the website, which then in turn tweets the message on your behalf. Use at your own discretion.