Bring a large pot of water to a boil and cook the fettuccine according to package directions; do not add salt to the pasta water.
Meanwhile, place the butter and cubed cream cheese in a medium heavy-bottomed saucepan over low heat and warm gently until melting.
Add the Parmesan and heavy cream to the saucepan and stir frequently; whisk briskly if lumps form until the mixture is smooth.
Stir in the garlic powder, cracked black pepper, and dried oregano; keep the heat low and stir often to prevent sticking, reducing heat if the sauce begins to stick.
Taste the sauce and add salt only if needed; then dip a cooked noodle into the sauce and taste the noodle-plus-sauce to confirm seasoning, adjusting salt as required.
Drain the cooked fettuccine, return to the pot or a warm serving bowl, pour the sauce over the noodles, toss gently to coat, garnish with fresh oregano, and serve immediately.