The Old Boys Union (OBU) of Dharamaraja College has a long and proud history of supporting and nurturing the development of the school while establishing a national and international network of past Dharmaraja students united by the Rajan spirit. As far back as the 1950s the OBU maintained a register of the college past students based locally and internationally and drew on their expertise for the development of the school.

Today the OBU is a multifaceted organization with branches in USA, UK, Australia and is a significant strength to the Principal and the school administration. The OBU sees its role as one of a supportive organization which seeks to build on and further enhance Dharmaraja College standing as a prominent educational institution in Sri Lanka. It also functions a vital link between the college and the wider world a role that has become increasingly possible due to developments in communication technology and ICT.

By positioning itself as a bridge between the college and a rapidly changing wider social reality, the OBU is able to provide advice, guidance and opportunities college students that will better position them to succeed as adults when they complete their secondary education and enter society. Given the large number of past students in diverse fields of expertise the OBU is also able to use its network to support the Principal and the teaching staff in numerous ways and also to provide various services to the administrative and teaching staff of the school. The OBU has also played an active role in supporting the welfare of retired Dharmaraja staff by organizing health camps, recreational activities and religious observances to enrich the lives of teachers who nurtured generations of Dharmaraja students.