Peel and slice the garlic cloves thinly and set aside.
Bring a large pot of water to a boil, salt generously, and cook the squid ink pasta according to package directions; reserve 1 cup of the pasta cooking water, then drain the pasta.
Heat a large frying pan over medium-high heat, add the olive oil, then add the shrimp and cook about 1 minute per side until pink and just cooked through.
Add the butter cubes, sliced garlic, dried oregano, and chili flakes to the pan; as the butter melts, pour in 1/4 cup of the reserved pasta water and simmer gently for about 2 minutes, seasoning with salt and pepper.
Toss the arugula or baby spinach into the sauce and allow it to wilt briefly.
Add the drained pasta to the pan and stir to combine, adding more reserved pasta water as needed to loosen the sauce; taste and adjust seasoning.
Serve immediately, topped with grated parmesan if desired.
