When McMillen placed the Irish tricolour in the window of his election office in the lower Falls area, this sparked a riot between republicans, loyalists and the Royal Ulster Constabulary (RUC).
On the same night, Wednesday, 28 September 1964, a large force of the RUC armed with rifles, Sten guns, batons and crowbars smashed down the doors of McMillen's election HQ and removed the tricolour.
The following day the IRA replaced the flag in the window and police attacked a crowd who had gathered to support McMillen.
He prominently displayed in his election offices a verse of a poem by John Frazier, a Presbyterian from Co Offaly: "Till then the Orange lily be your badge my patriot brother.
The NICRA's peaceful activities resulted in violent opposition from many unionists, leading to fears that Catholic areas would come under attack.
In May 1969, when asked at an IRA army council meeting by Ruairí Ó Brádaigh how many weapons the Belfast Brigade had for defensive operations, McMillen stated they had only one pistol, a machine gun and some ammunition.
McMillen's role in the 1969 riots was very important within IRA circles, as it was one of the major factors contributing to the split in the movement in late 1969.
In June 1970, McMillen's Official IRA had their first major confrontation with the British Army, which had been deployed to Belfast in the previous year, in an incident known as the Falls Curfew.
In addition 35 rifles, 6 machine guns, 14 shotguns, grenades, explosives and 21,000 rounds of ammunition, all belonging to the OIRA, were seized.
Some OIRA members under McMillen's command, including the entire Divis Flats unit, defected to the new grouping.
McMillen, in response was accused of drawing up a "death list" of IRSP/INLA members and even of handing information on them over to the loyalist Ulster Volunteer Force.
The first killing came on 20 February 1975, when the OIRA shot dead an INLA member named Hugh Ferguson in west Belfast.