Sri Lanka is a country in South Asia that has a high-level health care system. The National Hospital, Sri Lanka (NHSL) situated in Colombo, is the largest teaching hospital in Sri Lanka and the final referral centre in the country consisting of 3000 beds and the bed occupancy rate is 75 with 3.7 average lengths of stay. Outpatient care, sometimes called ambulatory care, is defined as medical care or treatment that does not require an overnight stay in a hospital or medical facility. The outpatient department (OPD) is considered as the window to hospital services and a patient's impression of the hospital begins at the OPD. The OPD of the National Hospital of Sri Lanka (NHSL) also faces the problem of congestion. This study aims to identify the causes of congestion at OPD of NHSL and provide solutions. Key informant interviews with relevant stakeholders, focus group discussions, desk review of secondary data, and direct observations of OPD processes were used as methodologies. Key informant interviews were conducted with the deputy director, OPD, medical officer-in-charge, and nursing sister in charge of the unit by the principal investigator. Two focus group discussions were conducted, with 12 randomly selected medical officers and 10 nursing officers with a minimum of one year of work experience at the OPD. Data retrieved from the desk review of the secondary data from the admission book, and OPD monthly statistics. Direct observation was used to triangulate the data. Around 690,000 patients are treated annually in OPD of NHSL. The average number of OPD patients per day is about 1900. Usually, a doctor at OPD of NHSL examines 36 patients per hour. Therefore, consultation time is limited to 1.6 minutes which is highly inadequate. Root causes for congestion were analyzed by using the Ishikawa diagram. Poor layout arrangements, delay of doctors and quota system of doctors, the non-availability of the patient information management system, absence of proper referral system in the country, the larger number of staff members were identified as root causes. Improvement of the layout of OPD in a unidirectional way, advised doctors to start duties on time abolish the quota system of doctors, establishment of the computer-based patients' registration system, establishing laboratory within OPD, establishing two more counters at OPD pharmacy were the major recommendation of this study to minimize the congestion at OPD, NHSL.