"Didn't really get a chance to breathe first full day on the job," he told Eyewitness News Anchor Sade Baderinwa in his One Police Plaza office.
He quickly learned the seriousness of leading the United States' largest municipal police department, comprised of 77 patrol precincts, 36,000 officers and 19,000 civilian employees, according to the NYPD.
The NYPD's top cop is a plainspoken man with humble Irish-Catholic roots. He began his career as a transit cop in the early 80s and worked his way up the ranks. Now as commissioner, he starts each day at 4:45 a.m. and is at work by 5:30 a.m.
MORE: Walking New York City's most dangerous street with NYPD Commissioner James O'Neill
Stepping inside O'Neill's office, it's easy to see what's important to him. The walls and shelves are decorated in reminders of family, including pictures of his sons, Danny and Christopher, and art painted by his 86-year-old mother, Helen.
Helen was by his side with a bouquet of flowers the day her son was sworn in as commissioner, but O'Neill said she was not always supportive of his career choice.
"She'll deny this, but for a fact, she didn't speak to me for almost a year," he said. "She was that furious with me."
Nonetheless, he said his mother's influence is undeniable.
"She told us all growing up, 'It's not about you. It's about helping other people,'" O'Neill said.
O'Neill spends his little bit of free time cruising on his motorcycle, and sometimes, the die-hard Rangers fan suits up to hit the ice.
He's number 34 on an all-NYPD team, playing for more than a decade.
"Sometimes as a commissioner, people are hesitant to speak with you," he said. "But when you're playing hockey, not so much."
The commissioner was raised in Flatbush, Brooklyn, by a tight-knit, working class family, where his mother ruled the roost and church on Sundays was a must.
"It seems like everybody on the block had at least three or four kids," he said. "All we did was play outside. There's a sewer plate that used to be home plate when we used to play stick baseball."
Being the middle child, O'Neill sometimes got lost in the shuffle, but early on, O'Neill figured out how to blaze his own trial, and he's been doing it ever since. He said he's not concerned about what he leaves behind.
"I'm not a big legacy person," he said. "When I walk out the door, if I leave this city in a safe place than when I came here, then I'll be happy."
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