When the onus is on some party in a negotiation, the point is to say which of several possible parties really needs to make a move. People have been pushing the onus back and forth in the Brexit negotiation:
EU Says Onus Is on U.K. for Brexit Progress
The onus is on Parliament now to protect our post-Brexit rights
Philip Hammond sees Brexit onus on Europe
But now Theresa May has announced at an EU summit that
the onus is now on all of us to get this deal done.
While I grant that her claim seems orthographically undeniable — onus = on us — I wonder what the prime minister could possibly be talking about. There literally are only two parties to the Brexit negotiation, the UK and the EU, so who else could the onus be on? Or is “us” her fellow heads of government in Salzburg, who have the responsibility to take the decision out of the hands of the bumbling bureaucrats of Brussels?