Groundwork for landmark moment laid by former MLA
The passing of an Opposition Bill by an independent Assembly member who has since lost his seat has paved the way for one of the most momentous changes since political power-sharing was established.
For years, the Ulster Unionists wrestled with sharing power in a coalition with a Sinn Fein that retained connections to an armed IRA.
It so badly split the UUP that the DUP became the main unionist party.
Now, the UUP has left the big tent it helped create in a stated return to "normal democracy" after years of opposition from across the ministerial table.
The role of Leader of the Opposition is certain to go to Mr Nesbitt. The Opposition will be entitled to chair the powerful Public Accounts Committee (PAC), which scrutinises government spending.
There will also be Opposition Day debates on topics likely to hurt the Executive and additional speaking rights.
John McCallister, a former Ulster Unionist who left for the short-lived NI21 and latterly sat in the Assembly as an independent, lost his seat at last month's election.
His last substantive work was to secure the backing of Assembly members for a Private Member's Bill paving the way for an official Opposition.
The Bill was backed by Assembly members but opposed by Sinn Fein as a "Frankenstein's monster" that could not be controlled.
This will be the first Opposition at Stormont since 1972, when power was taken over by direct rule ministers.
Mandatory coalition rules were introduced in 1998 to enshrine the protection of minority voices in a society emerging from conflict.