Some folks are on the lookout for fine ribs, and others on the trail of bluegrass music. O'Brien's in Bethesda has both.

The other night the bluegrass band wore fishing caps. They were the Montgomery County Ramblers, rambling through Flatt & Scruggs favorites before a small but clearly appreciative audience.

Clearly, because at O'Brien's you have to put down your ribs to clap, and clap the customers did. Those without free hands whistled. Some, wearing cowboy hats, were friends of the band. And some were regular customers, come to eat juicy barbecue, beans and slaw.

In the saloon-like room every little table had its pile of gnawed bones and used napkins, lending additional informality to the scene. O'Brien's is, after all, a cafeteria.

Some people, says Jim the bartender, eat there every night -- one night the ribs, one night the chicken, always that slab of Texas toast on the side. "That's what they use down in Texas," explained a man serving ribs behind the counter, "to sop up the sauce."

The band plinked and sang on. In the background was the regular thump of someone chopping barbecued ribs apart with a cleaver -- in time to the music, of course.

Pushing back some tables to make an informal stage, the restaurant owners bring in a band on Monday, Tuesday and Wednesday nights, sometimes holding open mike on Wednesday. The rest of the week O'Brien's (which opened last year and is run by the same O'Brien family of the Rockville barbecue establishment) is too busy and needs the space for diners.

Sure it's honky tonk, and maybe the fake gas lamps burn a little too brightly. But it's a low-key, low-pressure evening in the lower level of the Air Rights Building.

Cowboy hats optional, toothpick required. O'BRIEN'S PIT BARBECUE -- 7305 Waverly Street, Bethesda. 654-9004. Live music this month: Mondays (except May 30) from 8 to 11, Tuesdays from 6 to 9 and Wednesdays from 8 to 11; open mike, May 18.