The ODM Director is among the leaders of the People's Assembly announced by NASA last month.
The Orange Party took to their official Twitter handle to to relay the news to their supporters.
The party failed to disclose the cause of his illness.
However, they re-assured the party's supporters that the senior ODM official was responding well to medication.
The ODM Party Secretary General Dr Agnes Zani were among senior party officials, family and friends who visited Ong'wen earlier on Thursday afternoon.
Ong'wen is among the leaders of the People's Assembly announced by NASA last month.
The team is charged with the responsibility of steering the coalition's People's Assemblies across the country.
Other members of the team include Economist David Ndii, Peter Mathuki, Hamida Kibwana, Judy Sijeny, Dr Mutakha Kangu and Koitamet Ole Kina.