Concentrated oxygen product gas is produced and delivered to a patient. Product gas is produced by (a) introducing ambient air into a first end of a column containing an adsorbent material that preferentially adsorbs nitrogen and removes oxygen-rich product gas out a second end of the column; (b) evacuating the column through the first end to remove adsorbed gas from the column; (c) repressurizing the column by introducing ambient air into the first end of the column until the column is near 1.0 atmosphere; and (d) repeating steps (a)-(c). Product gas is delivered as needed to the patient.