No one in Baltimore needed an explanation why a teary-eyed and emotional Michael Phelps, who had just won his 20th gold medal, suddenly burst into laughter atop the podium at the 2016 Rio Olympics. But the rest of the world did.

What was so funny? This is how he explained it to NBC afterward: My boys from Baltimore were down on the other end, and back in Maryland, we all say “O!” for the Orioles during that part of the National Anthem. And all of the sudden I hear them roar “O!” and I knew exactly where it came from, and I just lost it…

Anyone who has ever attended Camden Yards or M&T Bank Stadium—or run in the St. Patrick’s Day 5K, for that matter—is familiar with Baltimoreans shouting “O!” at sporting events in tribute to our beloved Birds during the playing of “The Star-Spangled Banner.” But most probably do not know the tradition’s origin story.

It dates, not surprisingly, to the rowdy, it’s-not-over-until-it’s-over summer of 1979. Sparked by a league-leading 48 come-from-behind victories, the Orioles topped all of baseball that year with 102 victories. On 11 occasions, they won after trailing by least three runs. Those O’s also delivered five ninth inning rallies, including a special night on June 22 when Doug DeCinces blasted a two-out, two-run, walk-off homer that sent Memorial Stadium into delirium and launched a phenomenon officially coined “Orioles Magic.”

However, the untold story behind shouting “O!” during the National Anthem did not emerge until many years later. Not until Memorial Stadium’s legendary Section 34 diehards gathered for a reunion after superfan Wild Bill Hagy passed away in 2007.

A local reporter began inquiring how it all started and the Section 34ers pointed the finger at a blonde, 5-foot-2, then-24-year-old Northern High School grad who rarely missed a game. Now 70 and still a diehard, Mary Powers spontaneously yelled “O!” during the first word of the second-to-last line—as in, “O! say does that star-spangled banner yet wave”—completely upsetting the normally solemn hush of the moment. And literally upsetting some patriotic fans in the upper deck around her.

“It was warm that night and I’m pretty sure it was a Friday because there was a big crowd,” recalls Powers, a retired financial services assistant. “A lot of people turned and shot me dirty looks, you know, they didn’t think it was appropriate.”

A local reporter began inquiring how it all started, and the Section 34ers pointed the finger at a blonde, 5-foot-2, then-24-year-old Northern High School grad who rarely missed a game.

One person who did not shoot Powers a dirty look was Hagy, the beer-guzzling, cowboy-hatted Dundalk cab driver famous for leading the O-R-I-O-L-E-S cheers from his Section 34 perch. He glanced back at Powers and smiled.

So did a crazy, then-26-year-old fan named Marty Bass, who confirmed this account shortly before retiring last month after a singular 48-year Baltimore broadcasting career.

“Mary was standing right behind me when I heard her scream ‘O!,’” reiterates Bass, who remains friends with Powers to this day and eventually got her on camera a few years ago to recount the saga.

“We knew Marty as the morning TV guy, but nobody ever treated him any differently than anyone else,” Powers says. “Just one of the group.”

She adds that before most games, she, Bass, and much of the Section 34 crew would pregame nearby at Waverly’s Olympic Lounge, which put out a big deli buffet, especially for the late 8:05 Friday starts. As The Washington Post once noted, not only Memorial Stadium but Waverly itself was colorful—“an interesting mix of working-class blacks, aging socialists, artists, retirees and renovating couples”—with not one but two sports bars just north of 33rd Street and Greenmount, the Olympic Lounge and the Stadium Lounge.

Powers explains the inspiration behind her now‑iconic slice of Baltimore history wasn’t anything she’d thought about beforehand. That summer, she and her baseball pals had simply gotten into the habit of accentuating the “O” at the start of all kinds of words— “O”-ver, “O”-nly, “O”-lympic Lounge, etc.—a jokey way of celebrating the team’s rebound from fourth place the year prior.

Hagy picked up on it the next night and slowly but surely it moved from section to section to the point where it could be heard during broadcasts that fall of the World Series (the results of which will not be discussed here).

Of course, it’s coincidental the pronunciation of “O” is a signature feature of Bawlmerese and that “The Star-Spangled Banner” was penned under bombardment in the city’s historic harbor.

Over time, yelling “O!” didn’t merely catch on, it morphed into a uniquely Baltimore statement of hometown pride.

“But don’t overthink it,” Bass says, winkingly. “It was, and is, just a little fun thing we do.”