'O'Brien says he was packing up his TV gear two weeks ago, when a heavy case fell on his forearm, causing it to swell up immediately.'
It happened while he was on assignment in Asia. Two days after what initially appeared to be a minor injury, O'Brien had to have emergency surgery that involved amputating his arm.
Doctors said he had acute compartment syndrome - which is essentially an increase in pressure that causes the blood to stop flowing to a certain part of the body.
O'Brien appeared on the "TODAY" show to talk about the incident. He says when it first happened, he didn't think much of it.
'When I think of all the risky things I've done in my life, jumping out of airplanes, flying airplanes, scuba diving, the fact that a heavy equipment case could take my arm off is mindboggling.'
But he says doctors ended up making a life-or-death decision.
'I lost blood pressure during the surgery due to the complications. The doctor made a real-time call and amputated my arm just above the elbow. He later told me it all boiled down to a choice ... between a life and a limb.'
O'Brien didn't return home after the incident. Instead, he immediately went back to work, reporting on the anniversary of the Japanese earthquake.
'I just dove right into work and it turned out to be a pretty good tonic for me.'
O'Brien is currently in rehabilitation. He plans to get a prosthetic attachment so he can get back to riding his bike, flying planes and shooting his own video.