This study aimed to clarify the effectiveness of post‐operative oral care, including tooth brushing, denture cleaning and tongue cleaning, after digestive tract surgery. Subjects included 30 elderly patients aged 60–98 years (74·9 ± 7·8 years) who underwent digestive tract surgery. Subjects were randomly divided into an intervention group and a control group. In the intervention group, the following oral care was provided daily over a 5‐min period in the morning starting at baseline (day of surgery) and continued for 5 days: gargling with povidone iodine, tooth brushing, denture cleaning using a special brush and tongue cleaning using a tongue brush. In the control group, the subjects only gargled with povidone iodine. We compared the following variables between the two groups at baseline and 5 days later: the sensation of dry mouth, intra‐oral gas concentration, pulmonary sounds, body temperature and bacterial flora. The number of patients with abnormal pulmonary sounds (dry or moist rales) increased from 1 to 2 in the intervention group and from 0 to 4 in the control group (P < 0·05). The average number of bacterial species per subject for the control group was 3·64 ± 1·34 pre‐operatively and 3·50 ± 1·74 post‐operatively, whereas that for the intervention group was 3·08 ± 0·95 pre‐operatively and 2·62 ± 0·65 post‐operatively. In the intervention group, there was a significant decrease in the number of bacterial species (P < 0·05). These findings indicate that post‐operative oral care in elderly patients undergoing digestive tract surgery lowers the number of bacterial species found in the oral cavity. This effect, in turn, might improve respiratory function.