Protestors gathered at Woodvale Park in the Shankill area and shouted abuse at the mayor as he was surrounded by officers trying to ensure his safety, pushing and shoving to get to him.
Nine officers were injured during the scuffles, sustaining cuts and bruises.
Mr Ó Muilleoir was checked over in hospital and said that he was just "sore in a few spots".
Damage was caused to his vehicle when missiles were thrown by the crowd.
North Belfast Area Commander Chief Inspector Andy Freeburn hit out at those responsible for what he branded an "appalling incident".
The PSNI will be investigating this appalling incident with a view to identifying those responsible.
PSNI Chief Inspector Andy Freeburn
"This should have been a positive day about the opening of a children's play park in the area, but instead was marred by disorder which has left a number of our community officers injured," Chief Inspector Freeburn said.
Mr Ó Muilleoir was attending the event to plant a tree and mark £2m in investment having been spent by Belfast City Council and the Department of Social Development.
But one local resident told UTV that the protest was being held because the Sinn Féin politician was opening the park in "a protestant area".
Referring to the decision not to allow a Twelfth of July parade to pass the Ardoyne shops, he added: "We say there is no shared space. We can't go where we want to go, so why is he able to come here and do whatever he wants?"
A local woman said: "This park is a shared space - anybody can come to it obviously.
"But whenever they heard that the Lord Mayor was coming to open the park, a Sinn Féin councillor, then that's when anger grew."
You cannot trample all over people's sensitivities and then a matter of weeks later, because you have a chain around your neck, waltz into a community pretending you represent everyone.
William Humphrey, DUP
Mr Ó Muilleoir later took to Twitter to state that a "minority of people will not drag Belfast back".
He also added his thanks to police officers that he said were assaulted at Woodvale while he was carrying out his duties as First Citizen of Belfast.
"As Lord Mayor for all, I promised to visit every part of Belfast if invited," he said.
DUP North Belfast MLA William Humphrey said that the protest was a response to Mr Ó Muilleoir's part in Sinn Féin supporting the restricting of the Union flag at Belfast City Hall back in November.
"For the Lord Mayor to think he can come here and carry out an opening, when he and his party have been involved in a campaign against the culture of the unionist community - particularly the Orange culture - is simply not real."
A fun day had been planned to celebrate the opening of both Woodvale Park in the Woodvale Road area and Dunville Park on the Falls Road.
According to Sinn Féin Culture Minister Carál Ní Chuilín, the DUP's Nelson McCausland was welcomed at the west Belfast park.
A total of £4m has been spent by Belfast City Council and the Department of Social Development on revitalising the two sites.