Upon his return from the ASEAN Secretariat in Jakarta, Ong was appointed Ambassador-At-Large at the Ministry of Foreign Affairs, Singapore. At the same time, he was Director of the Institute of Policy Studies at the Lee Kuan Yew School of Public Policy in the National University of Singapore. Concurrently, he was Singapore's Non-Resident Ambassador to Iran.
On 18 July 2011, Ong took up his post as High Commissioner of Singapore to Malaysia.
Ong is a member of the Governing Board of the Human Rights Resource Centre, based at the University of Indonesia in Jakarta. He is also a Board Member of the International Rice Research Institute (IRRI) Fund Singapore and The Asia Foundation Singapore. Ong is in the Global Council of the New York-based Asia Society.