Speaking at a press conference after a Wesak Day celebration at the Malaysian Buddhist Associatino (Perak branch) hall here, Ong said he was saddened by Dr Mahathir's sudden announcement.
Barisan Nasional was currently facing a huge challenge and his decision would not benefit Umno and the coalition, said Ong, who is also Housing and Local Government Minister.
As a former Prime Minister, his words carried weight and would affect all levels of the people including party leaders, he said.
“We do not hope for any more problems and neither should we create more,” he said. “Our country needs stability and not uncertainty. His decision would paint a bad impression for the rakyat (citizenry) and foreign investors,” he added.
Lumut Umno division chief Datuk Dr Zambry Abd Kadir also urged Dr Mahathir to reconsider his decision.
“I feel very sad with Tun's decision to leave the party. It should not have happened in the first place,” he said.
Dr Zambry, who is also Pangkor assemblyman, said Dr Mahathir was a great leader who provided a sense of direction and ideas for the party and country.
The problem involved a former president and the present one, he said, adding that it so happened that the predecessor had appointed the current one.
“So it is a complex issue,” he said, adding that the current leadership must immediately find ways to overcome the problems in the party.
Dr Zambry pointed out that it was not unprecedented for former party presidents to quit Umno, adding that Dr Mahathir was the party's fourth president to leave the party.
The other three were the late Datuk Onn Jaafar, Tunku Abdul Rahman and Tun Hussein Onn, he added.
In PETALING JAYA, Umno Supreme Council member Datuk Seri Azalina Othman Said said Dr Mahathir's decision would not solve the problems faced by the party and would instead give more strength to the Opposition.
"At this time when the party is under stress, all parties including leaders and former leaders must work together to heal Umno so it becomes firm and strong," she said.
In MALACCA, Umno veteran and stalwart Tan Sri Aziz Tapa, 85, said that he was shocked at the news and was at a loss for words.
“How can it be when he (Mahathir) says that he loves Umno? I met him only few days ago in Johor and everything seemed okay. What he is doing is not right,” he said when contacted here on Monday.
Umno supreme council member Tan Sri Rahim Thamby Chik described the move as an "interesting but tragic development" for Umno.
"It's shocking news to me. If true, then I would say that it is an interesting yet tragic development for the party,” he said.
Reacting to Dr Mahathir's resignation, Higher Education Minister Datuk Seri Mohamed Khaled Nordin said, “This will weaken the party at a time when it is facing its enemies.”