Even if you've never met them before, you've seen Gil Faizon and George St. Geegland out and about town.

The late-in-life bachelors created by comedians Nick Kroll and John Mulaney and seen in Broadway's knee-slapping, hilarious new play "Oh, Hello!" — which opened Sunday – are Upper West Side legends.

They're the two guys squatting on the stationary bikes at the YMCA all afternoon, watching "Judge Judy" and rarely pedaling. Or the duo sitting on the crosstown bus, lecturing a standing pregnant woman holding a bag of groceries about the joys of Ed Koch-era New York.

Faizon (Kroll, of NBC's "Parks and Recreation") is a "Tony Award-viewing" actor who somehow has found his way onto your co-op board even though he probably doesn't live in your building. The bottom of his flowered shirt sticking out of the zipped fly of his pleated corduroy pants, Faizon calls himself "the kind of guy who brings beverages to the bathroom."

Also wearing pleated corduroy pants — though dressed up with a turtleneck, a cardigan and a blazer — is St. Geeland (Mulaney, of NBC's "Saturday Night Live"). He describes himself as "the type of guy you would catch at a party going through the coats." Neither Jewish nor a woman, he says that "like many old men over 70, I have reached the age where I am somehow both."

If the characters feel all too real, it's because they practically are. Kroll and Mulaney first spotted the two real-life inspirations for the fictional characters in 2008 while shopping at The Strand bookstore. Both men piqued Kroll and Mulaney's curiosity when they purchased separate copies of Alan Alda's autobiography "Never Have Your Dog Stuffed: And Other Things I've Learned."

The comedians followed the two down the block and across the street to a coffee shop, where each read their copies of the book separately. After eavesdropping on their conversation, Kroll and Mulaney brought Faizon and St. Geeland to the stage in a weekly show at the now-closed Rififi comedy club.

They've been performing the characters ever since — appearing regularly on the popular Comedy Bang Bang! podcast and in three seasons worth of segments on Kroll's 2013 Comedy Central series, "Kroll Show."

And now, they're on Broadway at the "deeply haunted" Lyceum Theatre — where discarded set pieces from 2003's "I Am My Own Wife," 2005's "Steel Magnolias" revival and some unnamed August Wilson play have been cobbled together as the setting for the 95-minute laugh-fest.

The show's plot is thin. Both are performing in a play written by St. Geeland, about two men who find fame on their NY1 celebrity prank show "Too Much Tuna" after being kicked out of their Upper West Side rent-controlled apartment, where they've lived as roommates for over 40 years.

But you're not there for the plot. You're there to see Kroll and Mulaney's masterful take on the Faizon and St. Geegland — two incredibly nuanced and well-rounded characters who manage to be wildly offensive and incredibly loveable at the same time. Both are equally excellent in the roles.

The laughs rarely slow down in the pitch-perfect show either. Built for an audience of "comedy nerd and theater dorks," its laughs will resonate strongly with New Yorkers and Steely Dan fans alike.

The limited engagement production also promises a celebrity cameo each night, brought onstage for the aforementioned "Too Much Tuna" prank in which the unsuspecting star diner receives an overstuffed Tuna sandwich. Sunday's matinee saw Grammy winner Josh Groban at the table. Other guests have included Rebel Wilson, Seth Meyers, Bobby Cannavale, Aziz Ansari and Fred Savage.

Alt-comedy fans will surely love "Oh, Hello." But rarely do theatergoers get to see such a refreshing, relatable character study. You'll be glad you said "Oh, Hello" to these two.