In a large pot or dutch oven, brown sausage over medium-high heat (about 10 minutes). Drain and discard grease. Lower heat and add onion and garlic, stir and cook for a few minutes.
Add potatoes, chicken stock, oregano, and red pepper flakes. Stir and bring to a boil, then reduce heat to low. Simmer until potatoes are tender, about 30 minutes.
Stir kale into soup. Season to taste with black pepper. Simmer until kale is tender, about 10 more minutes. Off the heat, stir in milk. Enjoy!