I'm not much of a sausage person, but I really really liked OSS.
OSS takes the simple hot dog/brat and makes it so much more. The creativity in their menu items is a bit overwhelming at first, because (at least in my case) you'll want to order everything. We tried the Korean BBQ and Doner Kebab signature sausages. The Korean BBQ one is a pork sausage topped with kimchi, tons of cilantro, and caramelized onions all nestled into a hot dog bun. I had jalapenos added as well. It was very tasty and unique. The Doner Kebab was also excellent. It had a lamb and chicken sausage with a Turkish-inspired cole slaw and tzatziki, among other toppings that I can't remember. My one complaint about the sausages is the buns they use, but that might just be a matter of preference.
We also ordered cheese curds and fries. The cheese curds here might be my favorite of anywhere in Madison. They are very lightly battered and fried, yielding a super light and delicious fried curd that feels way less terrible for you than it is. Waayy better than the grease-gobs you might find elsewhere. The fries were crispy and yummy, just the way we like them. The portions are a good size for the price ($2 for a ton of fries!), and OSS offers a great selection of beers on tap as well.