A process for producing acetic acid, comprising: allowing methanol to carbonylation react with carbon monoxide in the presence of a catalyst system, acetic acid, methyl acetate, and water, wherein the catalyst system comprises a metal catalyst, an ionic metal iodide, and methyl iodide; separating the reaction mixture into a volatile phase and a less-volatile phase; distilling the volatile phase to form a first overhead and an acetic acid stream, wherein the first overhead is rich in at least one lower boiling component selected from the group consisting of methyl iodide and acetaldehyde, and the acetic acid stream is rich in acetic acid; and at least one section selected from the group consisting of the following sections,, and: a purification section for obtaining purified acetic acid from the acetic acid stream; a separation section for separating at least acetaldehyde from the first overhead; and an off-gas treatment section for absorption-treating an off-gas from the process with an absorption solvent and forming a carbon monoxide-rich stream and an acetic acid-rich stream; wherein a concentration of oxygen is controlled in at least one selected from the group consisting of the following (a) and (b): (a) the concentration of oxygen in a gaseous phase of the process is controlled to less than 7% by volume, (b) the concentration of oxygen in a liquid phase of the process is controlled to less than 7 × 10 -5 g/g. allowing methanol to carbonylation react with carbon monoxide in the presence of a catalyst system, acetic acid, methyl acetate, and water, wherein the catalyst system comprises a metal catalyst, an ionic metal iodide, and methyl iodide; separating the reaction mixture into a volatile phase and a less-volatile phase; distilling the volatile phase to form a first overhead and an acetic acid stream, wherein the first overhead is rich in at least one lower boiling component selected from the group consisting of methyl iodide and acetaldehyde, and the acetic acid stream is rich in acetic acid; and at least one section selected from the group consisting of the following sections,, and: a purification section for obtaining purified acetic acid from the acetic acid stream; a separation section for separating at least acetaldehyde from the first overhead; and an off-gas treatment section for absorption-treating an off-gas from the process with an absorption solvent and forming a carbon monoxide-rich stream and an acetic acid-rich stream; wherein a concentration of oxygen is controlled in at least one selected from the group consisting of the following (a) and (b): (a) the concentration of oxygen in a gaseous phase of the process is controlled to less than 7% by volume, (b) the concentration of oxygen in a liquid phase of the process is controlled to less than 7 × 10 -5 g/g. a purification section for obtaining purified acetic acid from the acetic acid stream; a separation section for separating at least acetaldehyde from the first overhead; and an off-gas treatment section for absorption-treating an off-gas from the process with an absorption solvent and forming a carbon monoxide-rich stream and an acetic acid-rich stream; wherein a concentration of oxygen is controlled in at least one selected from the group consisting of the following (a) and (b): (a) the concentration of oxygen in a gaseous phase of the process is controlled to less than 7% by volume, (b) the concentration of oxygen in a liquid phase of the process is controlled to less than 7 × 10 -5 g/g. (a) the concentration of oxygen in a gaseous phase of the process is controlled to less than 7% by volume, (b) the concentration of oxygen in a liquid phase of the process is controlled to less than 7 × 10 -5 g/g.