We study the truncation functors and show the existence of projective cover with a finite Verma flag of each irreducible module in parabolic BGG category O over infinite rank Lie algebra of types a, b, c, d. Moreover, O is a Koszul category. As a consequence, the corresponding parabolic BGG category O over infinite rank Lie superalgebra of types a, b, c, d through the super duality is also a Koszul category.Notations. Throughout the paper the symbols Z, N, and Z + stand for the sets of all, positive and non-negative integers, respectively. All vector spaces, algebras, tensor products, et cetera, are over the field of complex numbers C.