A method for producing an oxide catalyst comprising Mo, V, Sb, and Nb, and a composition represented by the following formula: MoV a Sb b Nb c Z d O n wherein Z represents at least one element selected from the group consisting of W, La, Ce, Yb, and Y; a, b, c, and d represent values in ranges of 0.01 ≤ a ≤ 0.35, 0.01 ≤ b ≤ 0.35, 0.01 ≤ c ≤ 0.20, and 0.00 ≤ d ≤ 0.10, respectively; and n represents a value satisfying a balance of atomic valences, and wherein the oxide catalyst comprises 30% by mass or more and 70% by mass or less of a carrier based on a total amount of the oxide catalyst, the method comprising: a raw material preparation step of obtaining an aqueous mixed liquid comprising Mo, V, Sb, Nb, a carrier raw material and optional Z; an aging step of subjecting the aqueous mixed liquid to aging at more than 30°C; a drying step of drying the aqueous mixed liquid, thereby obtaining a dried powder; and a calcination step of calcining the dried powder, thereby obtaining the oxide catalyst, wherein, in the raw material preparation step and/or the aging step, precipitation of Nb is facilitated by performing at least one operation selected from the group consisting of the following to: in the raw material preparation step, the aqueous mixed liquid is prepared by mixing a Nb raw material liquid comprising Nb with a MoVSb raw material liquid comprising Mo, V, and Sb, wherein ammonia is added to at least one of the MoVSb raw material liquid, the Nb raw material liquid, and the aqueous mixed liquid such that a molar ratio in terms of NH 3 /Nb in the aqueous mixed liquid is adjusted to be 0.7 or more, and in the aging step, a temperature of the aqueous mixed liquid is adjusted to more than 50°C; in the aging step, a temperature of the aqueous mixed liquid is adjusted to more than 65°C; and in the raw material preparation step, the aqueous mixed liquid is prepared by mixing a Nb raw material liquid comprising Nb with a MoVSb raw material liquid comprising Mo, V, and Sb, wherein a molar ratio in terms of H 2 O 2 /Nb in the Nb raw material liquid is adjusted to less than 0.2, and in the aging step, a temperature of the aqueous mixed liquid is adjusted to more than 50°C. a raw material preparation step of obtaining an aqueous mixed liquid comprising Mo, V, Sb, Nb, a carrier raw material and optional Z; an aging step of subjecting the aqueous mixed liquid to aging at more than 30°C; a drying step of drying the aqueous mixed liquid, thereby obtaining a dried powder; and a calcination step of calcining the dried powder, thereby obtaining the oxide catalyst, wherein, in the raw material preparation step and/or the aging step, precipitation of Nb is facilitated by performing at least one operation selected from the group consisting of the following to: in the raw material preparation step, the aqueous mixed liquid is prepared by mixing a Nb raw material liquid comprising Nb with a MoVSb raw material liquid comprising Mo, V, and Sb, wherein ammonia is added to at least one of the MoVSb raw material liquid, the Nb raw material liquid, and the aqueous mixed liquid such that a molar ratio in terms of NH 3 /Nb in the aqueous mixed liquid is adjusted to be 0.7 or more, and in the aging step, a temperature of the aqueous mixed liquid is adjusted to more than 50°C; in the aging step, a temperature of the aqueous mixed liquid is adjusted to more than 65°C; and in the raw material preparation step, the aqueous mixed liquid is prepared by mixing a Nb raw material liquid comprising Nb with a MoVSb raw material liquid comprising Mo, V, and Sb, wherein a molar ratio in terms of H 2 O 2 /Nb in the Nb raw material liquid is adjusted to less than 0.2, and in the aging step, a temperature of the aqueous mixed liquid is adjusted to more than 50°C. in the raw material preparation step, the aqueous mixed liquid is prepared by mixing a Nb raw material liquid comprising Nb with a MoVSb raw material liquid comprising Mo, V, and Sb, wherein ammonia is added to at least one of the MoVSb raw material liquid, the Nb raw material liquid, and the aqueous mixed liquid such that a molar ratio in terms of NH 3 /Nb in the aqueous mixed liquid is adjusted to be 0.7 or more, and in the aging step, a temperature of the aqueous mixed liquid is adjusted to more than 50°C; in the aging step, a temperature of the aqueous mixed liquid is adjusted to more than 65°C; and in the raw material preparation step, the aqueous mixed liquid is prepared by mixing a Nb raw material liquid comprising Nb with a MoVSb raw material liquid comprising Mo, V, and Sb, wherein a molar ratio in terms of H 2 O 2 /Nb in the Nb raw material liquid is adjusted to less than 0.2, and in the aging step, a temperature of the aqueous mixed liquid is adjusted to more than 50°C.