i wonder how do desktop apps without any domain names use oauth? or is it not supposed to be used this way? if so what do i use? say for tumblr they have an authentication api so i will have to put the username and password in the url/query string?
i am thinking of using WPF/Adobe AIR. how does something like tweetdeck work?
You should start by reading about getting started with OAuth. Eventually, even a desktop application will open a browser window to authenticate the user - TweetDeck and other Twitter clients do this, as you've probably noticed.
Tumblr, in your example, doesn't use OAuth but rather basic authentication that is being performed via simple HTTP web requests.
Twitter doesn't want users entering their credentials into your application. So at some point the desktop app will need to open a browser window through which Twitter can authenticate their users and return an access token representing the user. From that point the desktop app can use the access token to represent the user in all subsequent API calls to Twitter.