"It's just so frustrating.
"Why do we do research? I mean, there's 15 or 20 years of science that show that the problem with cellphones is the conversation."
Prof O'Hare's area of expertise includes human cognition, human error and the processes and characteristics of decision making.
"The issue is about how conversation disrupts attention.
"That's the issue that makes the hands-free hand-held thing just a complete nonsense.
"It's irrelevant."
When announcing a November 1 ban on hand-held cellphones last week, Transport Minister Steven Joyce said hands-free phones would continue to be allowed because they did not cause the same level of distraction.
Prof O'Hare considers that, texting aside, the issue with cellphones in cars is not about the "physical manipulation" of phones.
"The fundamental issue here is that when your attention is focused elsewhere, then that detrimentally affects the driving task."
He considers that for a driver there is a difference between talking to a passenger and talking to someone on the phone.
"The front-seat passenger can adjust their conversation to the driver demands . . . whereas on the cellphone, of course, the person you are conversing with has no knowledge of the driving situation you are facing.
"It's a lot more intrusive."
Prof O'Hare said research in the past decade had shown that during phone conversations drivers tended to stare ahead rather than scan the road.
"But even when people are looking at something, they are not taking it in when their attention is on the phone, which is quite remarkable.
"It's called inattention blindness."
Prof O'Hare said that occurred because of the conversation and was nothing to do with holding a cellphone.
"When you are processing a conversation in your ear, it's actually affecting what you are seeing through your eyes.
"Quite remarkably, people can just not see a stop sign, for example.
"That's the danger."
Mr Joyce said allowing hands-free phones recognised many business and trades people depended on cellphones for their livelihood.
Prof O'Hare dismissed this as a "specious argument".
"Either you are going to pay attention to what the science shows you or you are not.
"Personally, I think that's a real cop-out."