In terms of sausages you will find nothing better in Madison, insofar as I've experienced anyhow, as OSS.
They have a wide variety of uniquely flavored sausage / garnish combos that are just amazing -- each in their own way and most evoking some particular national flavor.
This makes it a great stop for a group of people who just can't decided if they want Mexican, Indian, etc... as everyone can scratch their flavor itch of choice. Having tried much of their regular menu I can say that I have not once been disappointed by any selection (though I do have my favorites -- their "doner kebab", a lamb and chicken sausage with a house-made cucumber yogurt sauce is just AMAZING)
They are affordably priced, their sides are simple but also amazing, and if you are dining in they have spacious tables and even have board games onhand if you want to take your time with it.
While I'm not much of a drinker, these days, they also have a bar there, as well -- so they've got that going for 'em!