A process for producing an oxidized nanofibrillar cellulose hydrogel, wherein the process comprises oxidizing cellulose pulp fibers in the presence of hypochlorite as an oxidant and a heterocyclic nitroxyl radical as a catalyst, wherein the total amount of hypochlorite ions fed is at least 2.0 mmol/g based on dry weight of the cellulose pulp fibers, and wherein the oxidizing is conducted for 1 to 8 h; adjusting the concentration of the oxidized cellulose pulp fibers prior to disintegrating to a concentration that is equal to or less than 3 % (w/w); and disintegrating the oxidized cellulose pulp fibers to obtain an oxidized nanofibrillar cellulose hydrogel; wherein all steps of the process after oxidizing are performed under aseptic conditions and in a closed environment so that the oxidized cellulose pulp fibers and/or the oxidized nanofibrillar cellulose hydrogel is transported from one process step to the next using aseptic connections, and wherein the aseptic conditions include using components of a system for producing the oxidized nanofibrillar cellulose hydrogel that are configured to be in contact with the oxidized cellulose pulp fibers and/or oxidized nanofibrillar cellulose that are decontaminated and provide uncompromised, continuous isolation of their interior from the external environment; and wherein the oxidized nanofibrillar cellulose hydrogel obtainable by the process is sterile or essentially sterile and has a viscosity of at least 2000 mPa·s, as measured at a 0.8% (w/w) concentration of the nanofibrillar cellulose in water with a Brookfield viscometer at a temperature of 20 °C, with a vane spindle and a measuring speed of 10 rpm. oxidizing cellulose pulp fibers in the presence of hypochlorite as an oxidant and a heterocyclic nitroxyl radical as a catalyst, wherein the total amount of hypochlorite ions fed is at least 2.0 mmol/g based on dry weight of the cellulose pulp fibers, and wherein the oxidizing is conducted for 1 to 8 h; adjusting the concentration of the oxidized cellulose pulp fibers prior to disintegrating to a concentration that is equal to or less than 3 % (w/w); and disintegrating the oxidized cellulose pulp fibers to obtain an oxidized nanofibrillar cellulose hydrogel; wherein all steps of the process after oxidizing are performed under aseptic conditions and in a closed environment so that the oxidized cellulose pulp fibers and/or the oxidized nanofibrillar cellulose hydrogel is transported from one process step to the next using aseptic connections, and wherein the aseptic conditions include using components of a system for producing the oxidized nanofibrillar cellulose hydrogel that are configured to be in contact with the oxidized cellulose pulp fibers and/or oxidized nanofibrillar cellulose that are decontaminated and provide uncompromised, continuous isolation of their interior from the external environment; and wherein the oxidized nanofibrillar cellulose hydrogel obtainable by the process is sterile or essentially sterile and has a viscosity of at least 2000 mPa·s, as measured at a 0.8% (w/w) concentration of the nanofibrillar cellulose in water with a Brookfield viscometer at a temperature of 20 °C, with a vane spindle and a measuring speed of 10 rpm.