He spoke out after holding face-to-face talks with Theresa May in Downing Street aimed at kick-starting the stalled Northern Ireland peace process. I am wary about it.
The BBC reported DUP sources who said the talks were going well but it was thought "inappropriate" to make an announcement while events were unfolding at the Grenfell Tower in west London, where at least six have died.
She added that the British government has taken away £1billion from Northern Ireland.
He said: "Any deal which undercuts in any way the process here or the Good Friday and the other agreements is one which has to be opposed by progressives".
If a deal was to be delayed it would mean the Queen's Speech, which had originally been planned for next Monday, could be put back for at least a week.
She is set to meet with all 10 DUP members of parliament along with unionist and nationalist leaders.
"I will be making it very clear that any deal between the Tories and the DUP can not be allowed to undermine the Good Friday and subsequent agreements", O'Neill said.
A DUP source was quoted saying the party backed Mrs May's plan to leave the EU single market and the customs union, in a blow to efforts by Ruth Davidson, Philip Hammond and Tory back-benchers to push for a softer Brexit.
Former Prime Minister John Major earlier warned that a Conservative-DUP deal could create the perception that London was no longer an honest broker of the peace settlement in Northern Ireland.
Negotiations broke up on Tuesday night without an agreement, but Mrs May said the discussions had been "productive".
The pact with the DUP had been expected to be sealed yesterday after the prime minister said she had made significant progress in talks.
The DUP leader said: "There's been a lot of commentary around the issues that we are talking about and it won't surprise anyone that we are talking about matters that pertain, of course, to the nation generally".
However, the source added that a full deal on the government's programme wasn't required before the opening of Parliament, adding that there would be "steady dialogue" with the DUP.
"We're united Irelanders, we want to govern ourselves".
Mrs May stressed that Brexit would happen and the timetable remains on course.