We introduce twisted arrow categories of operads and of algebras over operads. Up to equivalence of categories, the simplex category ∆, Segal's category Γ, Connes cyclic category Λ, Moerdijk-Weiss dendroidal category Ω, and categories similar to graphical categories of Hackney-Robertson-Yau are twisted arrow categories of symmetric or cyclic operads. Twisted arrow categories of operads admit Segal presheaves and 2-Segal presheaves, or decomposition spaces. Twisted arrow category of an operad P is the (∞, 1)-localization of the corresponding category Ω/P by the boundary preserving morphisms. Under mild assumptions, twisted arrow categories of operads, and closely related universal enveloping categories, are generalized Reedy.