Health critic France Gelinas says the independent investigation would be open, transparent and the findings would be made public.
Ombudsman Andre Marin doesn't have jurisdiction over hospitals, but Gelinas says he can launch a probe if asked to by the government.
Premier Kathleen Wynne says an independent investigator will review quality assurance in Ontario's cancer drug supply chain.
The watered-down drugs were given to patients at four Ontario hospitals and one in New Brunswick, which is joining the review.
Wynne says it's unacceptable that it happened and promised to get to the bottom of it.
Many hospitals mix the medications themselves, but the five hospitals involved all used the same supplier based in Hamilton to prepare the drugs.
There was too much saline added to the bags containing the chemotherapy medications, in effect watering down the prescribed drug concentrations.
The five hospitals are contacting affected patients to arrange quick appointments with their oncologists.
It's a good first step, but more needs to be done, said Gelinas.
"In order to fix the problem, we need to know what happened in the first place," she said in a statement.
"Ontarians deserve to have complete confidence in their health-care system and the best way they will get that is if they get the answers as to what went wrong and how come it took so long to find the error."
Health Canada, the provincial governments, Cancer Care Ontario and the Ontario College of Pharmacists are investigating. The hospitals are also conducting their own probes.
The underdosing affected 665 patients at London Health Sciences Centre, 290 patients at Windsor Regional Hospital, 34 at Lakeridge Health in Oshawa, one patient at Peterborough Regional Health Centre and 186 patients at Saint John Regional Hospital.