OAKLAND — Scott Olsen has taken to YouTube to decry an arbitrator’s ruling that reinstated the Oakland police officer who tossed a tear gas grenade at a group of people who tried to help him as he lay injured during an Occupy Oakland protest.

“I think that throwing a grenade at people who are trying to help an injured person is one of the most malicious things you can do,” Olsen said of Officer Robert Roche. “He is clearly a sadistic person.”

Olsen, an Iraq War veteran, sustained a skull fracture and permanent brain damage when he was struck in the head by a lead-filled beanbag fired by a police officer who has never been identified during a 2011 Occupy Oakland protest. As Olsen was on the ground hurt, Roche is seen on the video tossing the grenade at him and several protesters who came to his aid.

The Police Department fired Roche over the incident, but on Wednesday arbitrator David Stiteler reinstated Roche with back pay, citing that Roche had been following orders, according to Roche’s attorney Justin Buffington.

The three-minute video released Friday notes that Roche committed three fatal shootings and includes social media posts purportedly from the officer.

In one post, Roche writes about his pending arbitration hearing, “Four more days until arbitration. It’s about (expletive) time. Shooters ready, stand-by, …”

Olsen, who received a $4.5 million settlement from the city, ended the three-minute video saying, “OPD has brought a serial killer back on the force. And in that act has let officers know that they can kill people and cost their city millions at the expense of the people of Oakland and then get paid vacation.”

In an email response to the video, Buffington wrote to this newspaper that an outside investigator hired by the city determined that Roche was unaware Olsen was on the ground when he tossed the grenade. Roche’s view was obstructed by his gas mask and the fact that he was looking “down the sights” of his beanbag gun, Buffington wrote.

Buffington also wrote that Roche’s prior shootings had been justified by investigators. He added that Roche had received “the highest possible rating” in his performance evaluations and that the only time he was disciplined was for using profanity. “That is an astonishing statistic given the way that OPD dispenses discipline like it’s candy,” Buffington wrote.

Contact Matthew Artz at 510-208-6435.