Cook shells in a large pot of salted boiling water, stirring occasionally, until very al dente, about 8-9 minutes; drain.
Meanwhile, heat oil in a large skillet over high heat. Add mushrooms and cook, stirring occasionally, until nicely browned, 5–6 minutes; season with salt and black pepper. Reduce heat to medium, add wine, and cook, stirring, until reduced by half, about 2 minutes. Add spinach, cover, and cook until beginning to wilt, 1–2 minutes. Uncover and continue to cook, stirring occasionally, until most of the liquid is evaporated, 2–4 minutes more. Transfer mushroom mixture to a large bowl.
In the same skillet, cook garlic and butter over medium-high heat, stirring occasionally, until garlic is fragrant and beginning to brown, 1-2 minutes. Add marinara sauce and crushed red pepper and bring to a simmer over low heat. Cook, stirring occasionally, until warmed through, 6–8 minutes.
While sauce is cooking, add ricotta, 6 tablespoons Parmesan, 2 tablespoons oregano, and remaining 1/2 tablespoon salt to mushroom mixture and stir to combine. Spoon about 2 tablespoons ricotta mixture into each shell.
Nestle stuffed shells into hot sauce in skillet. Cover and cook over medium heat until shells are warmed through, 4–6 minutes. Remove from heat and let sit 5 minutes. Top with Parmesan and remaining 1 tablespoon oregano.