If you hadn't heard him play, you'd never guess Hod O'Brien was a sparkling bebop pianist: his name sounds as if it belongs above a tavern door, while his rugged face and quizzical expression suggest a world-weary Irish novelist. And in fact the 65-year-old O'Brien remains obscure, even among jazz fans--of course, it can't have helped that he essentially dropped out of music for a decade just as his career was getting started in the 60s, studying mathematics and then earning a degree in psychology (and a subsequent research position at New York University). Even the retro-minded 90s somehow managed to pass him by, despite his perfectly authentic yet unquenchably fresh approach. Born in 1936, only 12 years after Bud Powell, O'Brien could hear bop and hard-bop piano from primary sources--Powell and Horace Silver, respectively--rather than on reissued recordings of their work or through fourth-hand iterations of their styles. He belongs to the second generation of boppers, the ones who developed the more relaxed, longer-form improvisations of the 1950s; his first significant gig came in 1957, with protomodern bassist Oscar Pettiford, and today he's among a handful of pianists (including Chicagoan Stu Katz) who truly inhabit the idiom as opposed to having merely studied it. O'Brien's spare, clean attack belies the complexity of his lines: they sound airy and effortless but contain an unexpected abundance of well-placed ornamentation, layered development, internal rhyme, and the like. Though his handful of discs over the last dozen years constitute a rather desultory return to recording, just one as good as his latest American release, So That's How It Is (Reservoir), would merit celebration. So does the end of his long absence from Chicago, the city of his birth: O'Brien performs here for the first time ever this week, in a one-night engagement with the matching-gloves rhythm team of bassist Kelly Sill and drummer Joel Spencer. Monday, October 15, 8 and 10 PM, Jazz Showcase, 59 W. Grand; 312-670-2473.