K.T. Oslin is a 48-year-old, no-nonsense kind of lady. She's done it all. Broadway actress. Madison Avenue jingle singer. Houston folkie. L.A. music adventurer. And three years ago she won her first Grammy.

It was for her debut album, "80's Ladies" (RCA), a compilation of female-spirited songs, sung rather assertively. The National Academy of Recording Arts and Sciences thought she gave the Best Country Vocal Performance in 1987. Not bad for someone who didn't even decide that she wanted to be a country singer and songwriter until she was, well, middle-aged.

"I'm glad it happened this late," she says from her bus outside of Rochester, N.Y., "because usually things are over at this point. I'm at a physical peak. I've got a stronger voice now than I had in my twenties and thirties, and it will stay that way if I take care of it properly."

There is a downside, though, to becoming a success as a mature woman rather than as a starry-eyed kid: "You are too wise. You don't buy into the magic of it all."

When Kay Toinette Oslin was in her twenties, she was searching for some kind of stardom. She first hit the stage in high school and earned a scholarship to a junior college in Texas, where she studied drama. In 1962 she moved to Houston to become a folk singer, but after a while she meandered back into acting, and eventually set out for New York, Equity card in hand.

During her 21 years there, Oslin landed some decent roles -- the Lincoln Center revival of "West Side Story," the touring company of "Hello Dolly" and the Burt Bacharach/Hal David musical "Promises, Promises," to name a few.

She says, though, that Broadway didn't give her "the thrill" she thought it would. So she started auditioning for commercials. She acted in a few and sang in a few. "It didn't feel right either."

Then, one day, she saw something scribbled on a bathroom wall: I ain't never gonna love nobody but Cornell Crawford.

"So I wrote a song," she says.

And it went from there.

Sissy Spacek, Gail Davies and Dottie West have all recorded Oslin compositions -- before Oslin did herself.

"I tried to hold on to them myself," she says, "because I had a feeling they were going to work."

Her feeling proved correct. When "80's Ladies" was released in 1987, it entered the charts at No. 15.

She says that what makes her songs a little different is that they aren't just dissertations about love or relationships. She works her theatrical background into them, adding a new dimension by creating characters who would say such things and feel such emotions.

"I've always thought that country music lent itself to theatrics even more than rock-and-roll," she says, "because it tells stories and deals with adult themes. When I sing a song or write a song, I try to make them unique. They take on a personality. They are a different person in each one."

K.T. Oslin is performing at the Kennedy Center Concert Hall Friday night at 8:30. Tickets are $16.50 to $22.50 and are available at all Ticketron and TicketCenter outlets or the Kennedy Center box office. For information, call 467-4600.