Mr Ong's move was reported on Tuesday by Lianhe Zaobao. But he had also remained silent on his political future in an interview with the Chinese daily, saying only that he was "looking forward to turning a new page in his career".
He described joining Keppel as a "personal career move", noting that he had never been in the private sector before, and said that one of Keppel's main draws was its international competitiveness.
Mr Ong was a high-flier in the civil service, taking on posts such as the Prime Minister's principal private secretary and the chief executive of the Singapore Workforce Development Agency before leaving the elite Administrative Service to join the National Trades Union Congress (NTUC) in 2008.
That paved the way for him to run as a PAP candidate in the 2011 General Election, when he was widely expected to be given a ministerial post. The loss of Aljunied GRC to the Workers' Party, however, appeared to have scuppered those plans.
Last November, Mr Ong stepped down as NTUC's deputy secretary-general, sparking much speculation about his next move. His name was also among several bandied about as a potential PAP candidate for a possible by-election in Punggol East.
The single seat was vacated after its MP, Speaker of Parliament Michael Palmer, resigned over an extramarital affair.
On Tuesday, Mr Ong told The Straits Times that he was maintaining his links with the labour movement as chairman of the Employment and Employability Institute and adviser to several unions.
"I continue to serve as a community and grassroots volunteer," he said, adding that all these were unpaid positions.
Just last Sunday, he ushered in the New Year with Kaki Bukit residents, in his capacity as adviser to the ward's grassroots organisations.
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