Will not enter the coalition and the movement "For the rule of law", headed by former Prime Minister Nuri al-Maliki. The results there, which may be delayed due to tensions between local parties, will not affect Sadr's standing.
Sadr, an opponent to both US and Iranian influence in Iraq, was followed by Iran-backed Shia militia leader Hadi Al Amiri's coalition.
His fearsome Mehdi Army militia led two uprisings against USA forces, whose withdrawal he consistently demanded, and was blamed for the killing of thousands of Sunni Muslims in the sectarian violence that plagued Iraq in 2006 and 2007.
According to a participant, the aim has been to unite Abadi and Maliki - bitter foes despite coming from the same Dawa party - alongside the Conquest Alliance, which looks set to come second in the election. Corruption has been at the top of Sadr's agenda for several years. The Associated Press says he has "in recent years sought to recast himself as a populist, railing against corruption and failing services". Since the first elections following Saddam's ouster, the Shiite majority has held the position of prime minister, while the Kurds have held the presidency and the Sunnis have held the post of parliament speaker.
But even then, his bloc might not necessarily form the next government.
Seats in parliament will be allocated proportionately to coalitions once all votes are counted. The government should be formed within 90 days of the official results. He is now calling for the latest deployment of American troops to leave following last year's defeat of the Islamic State group.
Only 44.52 percent of about 24 million people eligible to vote participated in the consultation, a decrease of 15 percentage points, compared to 2014. The Iranian-backed Shi'ite militia chief Hadi al-Amiri came in second with about 1.2 million votes and will control 47 seats.
A document provided to Reuters by a candidate in Baghdad that was also circulating among journalists and analysts showed results from all 18 provinces.
On Sunday night, officials from Iraq's election commission announced the nearly full returns from 10 of Iraq's 19 provinces, including Baghdad and Basra.
The remaining uncounted ballots, mostly from Iraqis overseas, the security services, and internally displaced people voting in camps and elsewhere, might change the final seat tallies but only marginally.
O'Neill warned that such a situation could be tenuous if America's former "enemy number one" is able to choose the next prime minister.