A witness said they still "can’t get over" seeing the alleged attack, which happened after a collision between a bicycle and car at the junction on South Hill this week.
"It was just horrific, a horrific example of road rage."
The witness said the young female driver had wound down the window to apologise to the cyclist.
"I heard her say, ‘I’m sorry, I didn’t see you’. Even I don’t know where the hell he came from.
"He was pummelling her and I started yelling ‘stop, stop’, because you know, you don’t hit women — and next he grabs the door and starts getting into her vehicle and he’s really getting into her.
"It was punches to the face, and to the temple and I was, like, ‘how can you do that?’ I mean, it’s beyond me."
Sergeant Blair Wilkinson said a cyclist had been charged with injuring with intent to injure after a "minor collision" between the bicycle and car.
He said the 66-year-old allegedly assaulted the 21-year-old female driver of the car in the incident in Wansbeck St at 9am on Monday.
Sgt Wilkinson said the woman was concussed.
The man will appear in the Oamaru District Court on October 1.
The witness said they were at the intersection of Wansbeck St (SH1) and Awamoa Rd when they saw the cyclist being hit by the car behind them.
"I had a very brief glimpse of a cyclist being hit and so I got out of the car to see what the cyclist was like, but before anything could happen, in the blink of an eye, he was round to the door of the driver and pummelling her with his ... fist.
"It was just horrific, a horrific example of road rage. I couldn’t believe it because he was an older chap, too."
The witness said they felt sorry for the young female driver, who was visibly upset and confused.
They said it was "heartening" to see so many people near the scene rush to the young woman’s aid.
"People came from all around. There was another lady in the queue of traffic and she made sure the young girl was OK and then she started directing traffic."
After the incident, the witness said they rang the police, while the cyclist fled on his bike, only to return just before police arrived.
"The horrifying part was this chap doing what he did. I still can’t get over it.
"I keep thinking, ‘what else could I have done?"’
While the cyclist tried to "justify" his actions to a growing crowd about what had happened, the witness stood firm.
"I said to them: ‘He’s just punched this woman repeatedly in the face and there’s no excuse for it’."
Once police arrived, they spoke to the man briefly before handcuffing him, putting him in the police car and taking him away, the witness said.
"Her father arrived shortly after and he was able to comfort her. It was just horrifying."
The witness said they had since been contacted by the mother of the female driver, who wanted to thank them for coming to her daughter’s aid after the incident.
"I understand she is doing OK. It was lovely that the mother rang to give her thanks," they said.
"It’s not what you want to see in the streets of Oamaru or anywhere, in fact."
