A coalition led by O'Neill's People's National Congress secured 60 seats in the 111-seat parliament, local reports said, with five still to be counted. No single party has ever won a majority and alliances are common in PNG politics.
"Now that we have formed government, we won't be discouraging the views that are different to ours," O'Neill told parliament today.
"We will be a government that listens more, talks less and works harder at every opportunity."
Many voters complained their names had been left off the electoral roll. Students torched ballot papers in the country's second largest city of Lae, protesting that not enough had been issued at their university.
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Polling was delayed for several days in the capital Port Moresby after officials went on strike over unpaid allowances.
One of O'Neill's main opponents, Don Polye's Triumph Heritage Empowerment Party, reportedly accused leaders of one constituency of "deliberately and openly rigging the election".
O'Neill, who won the last election in 2012, hailed this year's vote as "calm and peaceful" although monitors from the Commonwealth Observer Group called for a review of electoral processes, particularly given the number of vote-buying allegations.
The PNG leader has vowed to improve the quality of education, health care and infrastructure as well as reviewing the electoral process.