Pruaitch, also a former finance minister, thanked opposition bloc members for their support and said his team was ready for government.
"I want to assure PNG we do not have the time to waste. We will move quickly to ensure all systems are up and ready to deliver appropriate levels of services to citizens of our country."
O'Neill, who has been in power since 2011, on Sunday announced he was stepping down but has yet to officially resign.
He has nominated former prime minister Julius Chan, 79, to replace him and says he's confident the government will have the numbers to stay in power.
O'Neill was forced to announce his resignation after a series of ministerial resignations and MP defections from government ranks left him open to lose a vote of no confidence on the floor of parliament.
He has said defectors would return to government ranks after the leadership change.
"I am very confident that we will maintain a majority when we resume parliament on Tuesday," O'Neill said.
The opposition bloc, however, says it has more than enough members to unseat the government.
O'Neill has said he will visit the governor-general and officially resign this week.
But he has initiated Supreme Court proceedings to ensure "correct processes" are followed in any vote of no confidence against him, sparking speculation he is dragging things out to buy time to win back support and even stay in power.
Under parliamentary rules a no-confidence motion wouldn't go to the vote for more than a week.
Radio New Zealand reported on Tuesday that Chan has objected to O'Neill's court action and urged a prompt transition of power.
The defectors from the government allege O'Neill was a dictatorial leader and have raised concerns about the distribution of wealth from PNG's oil and gas resources projects.
Under scrutiny is O'Neill's involvement in a $A1.2 billion loan through finance group UBS for the government to buy a 10 per cent stake in Australian Stock Exchange-listed firm Oil Search which used the money to buy into the Elk Antelope gas field being developed by France's Total.
A report by the Ombudsman Commission of PNG criticising the 2014 deal and its lack of returns for Papua New Guineans is scheduled to be tabled in parliament next week.
Australian Foreign Minister Marise Payne on Monday said she was confident Canberra would maintain a good relationship with PNG no matter who leads the Pacific nation.