On a February Friday, a ping alerted me to the following email from a friend: "Hi! Oscar's has reopened after its short shutdown for health-code violations, so I figured we should go today for happy hour to celebrate. Dee has reserved a table for us."

That not even a chilling health inspector's report could deter us is, perhaps, the best indication of the deep affection Philadelphians feel for Oscar's Tavern, a portal from a newly swanky stretch of Sansom Street into the gritty, tobacco-stained heart of Center City in the 1970s.

Arguably the greatest dive bar in Philadelphia, which is — prove it isn't — the Dive Bar Capital of the World, Oscar's is what the sociologist Elijah Anderson might call a "cosmopolitan canopy."

This is common ground, a place where people of all races, classes, and genders treat one another with civility, union laborers huddled along the bar next to journalists, lawyers, and the unemployed. It is also the only bar where the staff remember my order: a pint of Philadelphia Brewing Co.'s Kenzinger kolsch and — this is the key to surviving a weekly bar column — a glass of water.

Oscar's opened in 1972 and, other than reluctantly quitting a nasty smoking habit, hasn't changed much since. There's still the acoustic tile ceiling, stained and perilously sagging, the ripped vinyl booths whose stuffing has been punished beyond any hope of resilience, and the table near the back with a lamp and an adding machine that constitutes manager Joe Mullan's office. Each table is still set with a crisp classic-cocktail place mat advertising drinks like the brandy Alexander and mint julep that are not, in fact, available for purchase here. "I think my restaurant supplier keeps them just for us," Mullan said. "We buy them a case at a time." (Customers do, from time to time, attempt to order off this menu and are gently steered toward, say, a 23-ounce Long Island iced tea instead.)

It's difficult to parse the appeal of this dim, narrow barroom tucked into a renovated rowhouse, but Mullan thinks it comes down to the staff. "We don't get much turnover," he says. He started working there when he was 21 and has put in 39 years; his wife works there. So does his sister-in-law. "It's like a neighborhood bar in Center City," he says.

I've been there for birthdays, engagement parties, and farewells. A friend hosted a bachelor party that stopped in there; the waitress brought a bottle of aspirin with the check. I went on one of the worst first dates of my life at Oscar's. But it lasted only 15 minutes before I retreated to a nearby table where some friends were sitting — because of course they were, because it's Oscar's — then debriefed with a waitress who declared the guy a well-known creep and alcoholic. I went on one of the best first dates, too: We went back to celebrate our anniversary a year later.

That's the reassuring persistence of Oscar's, a constant in a changing city.

"I get guys who come in and say, 'Holy Christ, you're still here?'" Mullan said. "Where am I going to go?"

Oscar's Tavern

1524 Sansom St.,  215-972-9938

When to go: Those times when you feel Philly just isn't what it used to be. It's open 11 a.m.-2 a.m. Monday-Saturday  and 5 p.m.-2 a.m. Sunday.

What to order: It's about quantity. Go for the tall, 23-ounce Yuengling ($4.50) and the cheesesteak ($8.25), which after 4 p.m. is actually a cheesesteak-and-a-half. (Oscar's, which once struggled to draw evening crowds in Center City's bleaker days, used to offer this special on all its sandwiches — turkey-club-and-a-half, burger-and-a-half, but that got a little messy.)

Bring: Dive bar connoisseurs, cheesesteak aficionados, and anyone who isn't too particular about what know-it-all health inspectors have to say.

Bathroom situation: It's nothing fancy, but the single-stall, reasonably well-maintained and lightly vandalized ladies room is also never as bad as I expect it to be.

Sounds like: A highly unpredictable TouchTunes medley, at a boisterous 90 decibels or so. Occasional, inexplicable sing-alongs have been heard toward the end of happy hour.