About six dolphins were swimming in the area at the time.
His picture created quite a bit of interest when he showed it around, Mr O'Callahan said, and would be featuring in the local residents' newsletter.
"It's a pretty spectacular photo with the two dolphins crossing over, one going clockwise, the other anticlockwise. It was very special."
He said dolphins could be seen swimming at Omaha Beach reasonably frequently.
"When there's a surf running, they sometimes surf in the waves."
Mr O'Callahan said the Whangateau Harbour in Omaha was one of the best preserved areas of water around Auckland, and was great for wildlife spotting.
It was relatively undeveloped and unpolluted, he said.
A seal had even been seen taking up residence there.
As for the other character in his photograph, Mr O'Callahan said he would love to know the boatie's identity. The man, who was also armed with a camera, may have some even better photos because he was so close, Mr O'Callahan said.
"I'd seen him a few minutes earlier with his camera on a tripod on the harbour side ... And then he suddenly packed up his tripod, jumped in his dinghy and went out to the heads."