The Irish Property Owners Association has said landlords are putting their properties up for sale because the rental market has been "distorted" by rent controls.
Mr O'Brien's clampdown on spiralling rents has resulted in landlords leaving the market, opting to cash in on their properties amid high house prices, shrinking an already tiny supply of homes for rent.
New Residential Tenancies Board (RTB) figures show the number of landlords exiting the market is now higher than pre-pandemic levels.
Opposition politicians have called on the Government to take action as the latest Daft.ie rental report showed average rents have risen by 5.6pc in the second quarter this year, which has been blamed on a shortage of properties.
Labour Senator Rebecca Moynihan said if landlords want to sell their rented-out properties, they should be sold with the tenant in situ.
"Renters need a break and Government needs to realise that people are renting homes, not investment opportunities," she said.
Sinn Féin housing spokesperson Eoin Ó Broin also said rented-out homes should be sold with the tenant in them.
"Sell the properties, that's no problem, but sell them with the tenant in situ," he said.
Mr Ó Broin said the Government needs to build more cost rental homes to meet demand for affordable accommodation.
Mr O'Brien has promised that 350 cost rental homes will be delivered this year.
However, opposition politicians have argued this is not enough.
Mr O'Brien launched his much-anticipated Affordable Housing Bill earlier this year.
However, a multi-billion euro housing plan, which was supposed to be launched in mid-July, has been delayed numerous times.