This study aimed to investigate the incidence of postoperative pneumonia (PP) and surgical site infection (SSI) in head and neck cancer (HNC) patients and clarify the relationship between oral care and postoperative infection. Methods: We conducted a retrospective observation survey based on the medical records of 209 HNC surgery patients managed at a University Hospital in 2016-2018. The incidence of PP and SSI were assessed in patients who underwent operations of the nose and paranasal sinuses to the larynx. Factors associated with PP and SSI in a univariate analysis were included in a multiple logistic regression analysis. A Cox proportional hazards model was used analyze the incidence of PP according to time after surgery. The present study was approved by the ethical review board of our Institute. Results: The rates of PP and SSI in our study population were 20.5% and 23.0%. Operative time (P < 0.01), blood loss (P = 0.004), tracheostomy (P < 0.01), reconstruction (P < 0.01), and preoperative plaque control record (PCR) (P < 0.01) were significantly associated with PP. The PCR depicted the oral hygiene based on the percentage of plaque attached to the tooth neck. A multiple logistic regression analysis indicated that the incidence of PP was significantly higher in patients with PCR values of ≥50% after preoperative oral care (OR = 10.174, 95% CI 2.14-48.32, P = 0.004). Tracheostomy (P < 0.01), reconstruction (P = 0.044), a lower preoperative albumin level (P = 0.019), and a lower preoperative hemoglobin level (P < 0.01) were significantly associated with SSI. Conclusions: The incidence of PP among patients who received oral care was high in those patients with high PCR values, indicating the importance of increasing compliance to preoperative oral care.