In 1955, Ong's foray into politics began as a polling agent for
Lee Kuan Yew in the
1955 legislative assembly election. In 1956, Lee Kuan Yew wrote to Ong and offered him a job as party organising secretary for the PAP. He stood for election in the 21 December 1957 for the fully elected
City Council of Singapore as a candidate for the
Tanjong Pagar ward and was successfully returned and became the first and only ever Deputy Mayor. He held the post until he resigned in April 1959 to contest the
Legislative Assembly election as the PAP's candidate for
Telok Ayer. This was to be the first Legislative Assembly in which all seats were elected. He won the seat, which he retained till his retirement in 1988. He was subsequently appointed as
Minister for Home Affairs in the first self-government
Cabinet and played a key role to eradicating yellow culture and crime in Singapore society. His cabinet appointment also made him part of the
Internal Security Council which sanctioned
Operation Coldstore in 1963. From 1963 to 1970, Ong took on the highly sensitive
Education ministerial portfolio at a time when
Chinese language culture and education issues were highly politicised. By increasing the teaching of
English in Chinese schools and vice versa, he was instrumental in laying the foundation for
Singapore's bilingual language policy. In 1970, Ong became the
Labour Minister. In 1980, he took over as the Environment Minister. In 1984, he stepped down from the Cabinet to make way for younger leaders. However, he displayed some unhappiness at the pace and manner by which he was sidelined from the political scene. He retired from politics in 1988 when his constituency of Telok Ayer was eliminated in the
3 September 1988 general election. Lee recognised Ong's displeasure in a public letter of appreciation: : "... I agree with you. You also had misgivings (about some newcomers), as had Chin Chye (referring to
Toh Chin Chye), over the speed of self-renewal and the effect it was having on the morale of the old guard MPs." Ong is considered as one of the "Old Guard", the first generation of leaders of independent Singapore. He is the sole remaining living member, after
Jek Yeun Thong's passing in June 2018. == Philanthropic work ==