A steel sheet for hot forming, consisting of, in mass ratio: C: 0.15 mass% to 0.35 mass%; Si: 1.0 mass% to 3.0 mass%; Mn: 1.0 mass% to 3.0 mass%; Al: more than 0 mass% up to 0.10 mass%; Ti: ( ×48/14) mass% to 0.10 mass% (where denotes the amount of N in the steel sheet); B: 5 mass ppm to 50 mass ppm; P: more than 0 mass% to less than 0.015 mass%; S: more than 0 mass% up to 0.010 mass%; N: more than 0 mass% up to 0.010 mass%; and O: more than 0 mass% up to 0.010 mass%, optionally, as other elements, in mass ratio: one or more elements selected from the group consisting of Cr: more than 0 mass% up to 1.0 mass%; Mo: more than 0 mass% up to 0.5 mass%; and one or more from among V, Nb and W in a total amount of more than 0 mass% up to 0.5 mass%, the balance being iron and unavoidable impurities, wherein the average oxygen concentration from an outermost surface of the steel sheet down to a depth of 10 μm in a sheet thickness direction is 0.70 mass% to 1.30 mass%, as measured by glow discharge optical emission spectrometry, wherein the measurement conditions include a measurement target in the form of a region having a diameter of 4 mm within the steel sheet surface, power of 50 W, argon gas at a pressure of 2.5 hectopascals, and the use of a dry glow discharge spectrometry model, with the measurement pulse set to 50%, and wherein the average oxygen concentration is calculated by working out a concentration profile of oxygen in the depth direction of a test piece having been cut out from the width central portion of a cold-rolled steel sheet having the above chemical composition, integrating the oxygen concentration in a region from a depth of 0 μm, to a 10 μm depth, and dividing the integral value by 10 μm. C: 0.15 mass% to 0.35 mass%; Si: 1.0 mass% to 3.0 mass%; Mn: 1.0 mass% to 3.0 mass%; Al: more than 0 mass% up to 0.10 mass%; Ti: ( ×48/14) mass% to 0.10 mass% (where denotes the amount of N in the steel sheet); B: 5 mass ppm to 50 mass ppm; P: more than 0 mass% to less than 0.015 mass%; S: more than 0 mass% up to 0.010 mass%; N: more than 0 mass% up to 0.010 mass%; and O: more than 0 mass% up to 0.010 mass%, optionally, as other elements, in mass ratio: one or more elements selected from the group consisting of Cr: more than 0 mass% up to 1.0 mass%; Mo: more than 0 mass% up to 0.5 mass%; and one or more from among V, Nb and W in a total amount of more than 0 mass% up to 0.5 mass%, the balance being iron and unavoidable impurities, wherein the average oxygen concentration from an outermost surface of the steel sheet down to a depth of 10 μm in a sheet thickness direction is 0.70 mass% to 1.30 mass%, as measured by glow discharge optical emission spectrometry, wherein the measurement conditions include a measurement target in the form of a region having a diameter of 4 mm within the steel sheet surface, power of 50 W, argon gas at a pressure of 2.5 hectopascals, and the use of a dry glow discharge spectrometry model, with the measurement pulse set to 50%, and wherein the average oxygen concentration is calculated by working out a concentration profile of oxygen in the depth direction of a test piece having been cut out from the width central portion of a cold-rolled steel sheet having the above chemical composition, integrating the oxygen concentration in a region from a depth of 0 μm, to a 10 μm depth, and dividing the integral value by 10 μm. one or more elements selected from the group consisting of Cr: more than 0 mass% up to 1.0 mass%; Mo: more than 0 mass% up to 0.5 mass%; and one or more from among V, Nb and W in a total amount of more than 0 mass% up to 0.5 mass%, the balance being iron and unavoidable impurities, wherein the average oxygen concentration from an outermost surface of the steel sheet down to a depth of 10 μm in a sheet thickness direction is 0.70 mass% to 1.30 mass%, as measured by glow discharge optical emission spectrometry, wherein the measurement conditions include a measurement target in the form of a region having a diameter of 4 mm within the steel sheet surface, power of 50 W, argon gas at a pressure of 2.5 hectopascals, and the use of a dry glow discharge spectrometry model, with the measurement pulse set to 50%, and wherein the average oxygen concentration is calculated by working out a concentration profile of oxygen in the depth direction of a test piece having been cut out from the width central portion of a cold-rolled steel sheet having the above chemical composition, integrating the oxygen concentration in a region from a depth of 0 μm, to a 10 μm depth, and dividing the integral value by 10 μm.