A million people could be waiting on an appointment in the public health system by the end of the year if non-Covid care cannot ramp up soon.
The Irish Medical Organisation has warned the capacity of the system may be halved to comply with social distancing rules.
It has called on the Government to build temporary facilities to make up for the capacity being lost when the takeover of private hospitals ends later this month.
He said 570,000 people are still waiting for an outpatient appointment - and a further 230,000 people are on a waiting list for an inpatient or day-case procedure.
Dr O'Connor said: "That's a function of how many appointments are conducted under normal circumstances that haven't been conducted.
Unless we get the system back up and running for routine care in the public sector, we could be looking at a million people by November or December.
"Our hospitals have been overwhelmed and operating at dangerous levels of capacity for many years and Covid has exposed the underlying fragility of our services.
"It is untenable that we continue with historic deficits in manpower and bed capacity in the context of increasing waiting lists."
Fellow IMO member and consultant psychiatrist Dr Matthew Sadlier warned of complacency about the outlook for further infections.
He said: "Until we have effective treatment options and a vaccine, we face continued uncertainty as to the impact of a second, and subsequent, waves particularly as respiratory illnesses begin to circulate again as early as September."
Dr Sadlier also urged the committee to invest in public hospitals, rather than through the National Treatment Purchase Fund (NTPF).
"The NTPF which was originally a short-term solution has become a long-term measure thus depriving the public system of investment and enabling the continued neglect of our health services.
"A policy that consistently diverts funding to NTPF in the absence of funding of our public health services will not address the problems of capacity and will not be a long term solution to waiting lists."