What made O'Day different from the jump was that she didn't sing like she needed your approval. Most band singers of the 1940s leaned into glamour and sentimentality — O'Day leaned into rhythm. She treated her voice like a horn, playing with the beat rather than just riding it. She came up with Gene Krupa's band and later Stan Kenton's, and those were demanding schools. You had to hold your own against loud, swinging ensembles, and she always did.

Her phrasing is the thing. She'd delay a note, clip another short, stress a syllable you wouldn't expect — and it always felt like a choice, not an accident. When bebop became the language of serious jazz musicians in the late '40s and '50s, she adapted without making a big deal of it. She didn't announce a reinvention. She just started singing differently, and it worked. Anita (1955) on Verve is a great place to hear this — it's a small-group record, relaxed but precise, and she sounds completely at home with the material in a way that a lot of vocalists chasing bop never quite managed.

Her Newport Jazz Festival appearance in 1958 is probably the single most-watched moment of her career, captured in the documentary Jazz on a Summer's Day. She performs in a polka-dot dress and a wide-brimmed hat, and the performance — especially her version of Sweet Georgia Brown — is a clinic in swing and cool happening at the same time. She's clearly having fun, but she's also absolutely locked in. People who see that clip for the first time tend to go looking for more.

Life off the bandstand was genuinely rough. She struggled with heroin addiction for years, and there were stretches where she was largely absent from recording. She was honest about all of it in her autobiography, which is a good read — direct and unsentimental, very much like her singing. The comeback records she made in the late '70s and into the '80s showed she hadn't lost the edge. The voice was a little different, but the rhythmic intelligence was still intact.

If you're putting together a first listen, start with Anita (1955) or Anita O'Day Swings Cole Porter with Billy May (1959). The Porter album is looser and more playful, and Billy May's arrangements give her room to stretch. Either record will tell you pretty quickly whether her particular brand of cool is going to click for you. For most people who care about jazz singing, it does.