He was educated at the Chinese High School in Singapore and proceeded to the University of Adelaide in Australia, to study architecture. He graduated with a bachelor's degree in architecture in 1961 and worked as an architect in Adelaide for two years before returning to Singapore in 1964. Ong worked as an architect in the private sector for nearly two years after returning to Singapore. In September 1965 he left for the United Kingdom to pursue a post-graduate degree in town planning at the University of Liverpool on a Colombo Plan scholarship. He obtained his master's degree in civic design (town planning) in 1967.
Ong returned to Singapore in 1967 and joined the planning department of the ministry of national development. However, he was soon sent to the United Nations Development Program (UNDP) special fund for assistance in urban renewal and development project, where he participated in the formulation of a long-term comprehensive concept plan for guiding the future physical development of his country. He had also led teams of planners in designing Telok Blangah new town and in conducting a comprehensive study of central area transportation and land use, with emphasis on the mass rapid transit (MRT). Ong left the civil service in 1971 to work as an architect and town planner in the private sector until June 1975, when he was appointed as senior minister of state for communications.
Ong's interest in community service began in the late 1960s when he assisted Hwang Soo Jin, then member of Parliament (MP) for Jalan Kayu, in his constituency work. However, his political career began after the September 1972 general election, when he was elected MP for Kim Keat constituency after winning 74 percent of the valid votes cast. Ong was among the first batch of ten People's Action Party (PAP) candidates elected into Parliament in 1972 to replace the old guard. After that he was returned as the MP for Kim Keat in 1976, 1980, 1984, and 1988. In 1991 the Kim Keat constituency became part of Toa Payoh Group Representation Constituency (GRC), but Ong succeeded in retaining his parliamentary seat.
After serving three years as senior minister of state for communications, Ong was appointed minister for communications in July 1978. He had been appointed acting minister for culture earlier in September 1977 and continued with this additional portfolio until 1980, when he was made minister for communications and labor. As minister for communications he had been concerned with the need for a mass rapid transit system and initiated detailed feasibility studies and visits to other countries. He succeeded in persuading his cabinet colleagues that Singapore needed the MRT system to improve its public transportation. During the launching of the MRT at Toa Payoh Station in November 1987, Ong said: "For me, this [the MRT] has been a 20-year love affair, from conception to delivery. "
In January 1981 Ong was elected chairman of the PAP central executive committee. In May 1983 Ong was made minister without portfolio and was elected secretary-general of the National Trades Union Congress (NTUC), the country's largest trade union. During the 1985 economic recession he was able to convince workers that wage restraint was necessary. In the same year he became chairman of the Singapore Labour Foundation (SLF) and, in that role, was responsible for setting up such social and recreational facilities as the NTUC Club, the NTUC Pasir Ris Resort, and the Orchid Country Club.
In 1985 Ong was appointed second deputy prime minister. In 1990 he became deputy prime minister. Thus, when Ong resigned from the PAP in August 1993 to contest the presidential election, he wore four important hats: deputy prime minister, chairman of the PAP central executive committee, secretary-general of the NTUC, and chairman of the SLF. What greatly interested Ong was amendment number 3 to the constitution, passed by Parliament on January 3, 1991. The new law changed the institution of the presidency from a ceremonial role to an elected position with powers over government budgets and key appointments in the public service.
In endorsing Ong as its choice in the presidential election in August 1993, the Straits Times, the major English-language newspaper in Singapore, wrote: "He has what it takes to shape the elected presidency in the nation's best interest. "
He married Ling Siew May in 1963. They had two sons.