In a statement, the department said it had launched an internal investigation into the incident inside the South Main Street Pub on April 26, which was caught on surveillance video.
The officer has been suspended without pay and his weapons have been seized, police said.
The bar employee in the video told Newsday Wednesday he was in total disbelief over the incident.
Charlie Ball said he didn't know the officer who walked into the bar with a mutual friend.
"I was turning around and filling a glass with soda and I didn't see it," Ball, 53, of Farmingdale, told Newsday Wednesday. "That's when he drew his weapon, pointed it at the back of my head -- and then I turned around and it's pointed at my face. I'm like, 'What are you doing? Put that away.' "
The video shows the off-duty officer holster the gun, then, later, pull it from the holster again, remove the clip and unload a round from the chamber. He then places the clip, the bullet and the handgun on the bar counter top.
"He pulled the weapon again, took out the ammunition clip and dislodged a bullet, put it all on the bar," Ball said. "I was in total disbelief . . . Later on, when I saw the tapes, it sunk in. I realized how dangerous it could have been . . . When I realized it was loaded it was more disbelief than anything. Every time I see it I realize it could have gone real bad. It could have gone real bad."
Ball said he works as a plumber and said he does clean up at the bar as a part-time job.
He said the man who arrived with the officer in question was a mutual friend.
He declined to identify the other man and police did not identify him, either.
Ball said that when the officer first pulled the gun the mutual friend said: " 'Dude, what are you doing? Put that thing away. What, are you out of your mind?' "
Ball said the mutual friend later told him: "I never expected anything like that."
Ball said that before the incident the off-duty officer was at times belligerent and, at one point, threatened to "come over the bar." Ball said he had noticed the gun and asked the man whether he was a cop before the officer pulled the weapon. Ball said the man identified himself as a Nassau police officer with 17 years on the job.
Ball said he debated whether to report the incident but later decided to contact police.
"My main concern," Ball said, "was that they get this guy and get his weapons away from him."
Calls to the Nassau PBA were not immediately returned Wednesday.