Footage shown in Southampton Crown Court showed O’Leary “walking casually” in the crowd in front of the police cordon when he spots a smoke grenade on the ground, picks it up and throws it towards police.
O’Leary also admitted resisting a police officer and possession of an offensive weapon – a samurai sword in his bedroom – when officers came to arrest him in the early hours of 7 June.
The court heard he adopted a “fighting stance” at the top of the stairs and threatened officers who had to use pava spray to subdue him.
From a search in his bedroom, officers also found the samurai sword, which O’Leary said he owned for 20 years as decorative only and he did not know the law had changed on owning one.
Meanwhile Connor Bishop, 24, from Southampton, was seen in footage wearing a black jumper with “boys get sad too” written on the back, carrying a yellow traffic cone which he threw towards officers.
He was seen running with the cone, “pursuing officers for some time with it”, prosecutor, Siobhan Linsley told the court.
“Once it’s thrown he then follows it again, picks it up again,” she said.
When he was arrested at his home on 3 June, he answered “no comment” but then accepted he was there when shown footage.
He also admitted throwing a box of screws and punching a wall which was not captured on footage.
He told a police interview he was brought up Christian and “threw items to fit into the crowd” and that he had drunk a small amount of beer “that made him more lairy”.
Defending Bishop, Thomas Evans said: “He is perhaps an example of peer pressure and group thinking”.
Evans said of Bishop and O’Leary: “They are not the instigators of this disorder, they are inevitable result of other individuals who seek to harness anger.”