No he hasn't!!!
O'Neill appointed Julius Chan as "caretaker Prime Minister" which under the PNG Constitution means nothing. Constitutionally, O'Neill must resign to the Governor General who sends the resignation on to the Speaker of Parliament. He may do that, but I doubt it.....
Parliament elects a Prime Minister: O'Neill has no authority to "appoint" a Prime Minister.
I suspect O'Neill is trying to frustrate the Vote of No Confidence (VONC) against him, which is due to be tabled in Parliament tomorrow? When it can't be tabled ostensibly because O'Neill says he is no longer PM, O'Neill will claim his Prime Minister position back from Chan.
Certainly shows Julius's colours!
Just another day in PNG politics.