Ong is an alumnus of Raffles Institution and graduated from the National University of Singapore in 1972 with a Bachelor of Science (with honours) in chemistry.

[1] Ong became a physiology lecturer at the Faculty of Medicine of the National University of Singapore (NUS) from 1975 till 1991.

[3] Ong was appointed Principal of Anglo-Chinese School (Independent) in 1994 when his predecessor, Lawrence Chia, stepped down after six years.

This was a significant step in the reform of Singaporean education system,[6] which had been cited by critics as being too harsh and rigid in its regimen, not to mention its examination-centric approach towards achieving academic excellence.

[7]The Anglo-Chinese School (Independent) publicity material for the International Baccalaureate Diploma Programme additionally emphasises that IB students guilty of serious violations of academic dishonesty, if male, may be caned.

However, his resignation came shortly after allegations of inappropriate behaviour towards a male teacher had been made about him to the ACS Board of Governors.

[12] Tan made statements to The New Paper stating that he filed the complaint because he "wanted justice to be served" and that there were "possibly a few people" involved in this matter.

[16] In April 2008, Ong underwent major open heart surgery to correct coronary disease.