Before the start of business on a recent weekday, droning vacuum cleaners erased the traces of customers at Prince George's Plaza, sales clerks methodically straightened racks of back-to-school dresses, and a dozen senior citizens paced in front of the locked entrance to Hecht's department store.

Shopping wasn't first on their agenda. When a clerk unbolted the front doors, the silver-haired crew hustled past the merchandise, down the escalator and into a spacious room behind the furnishings department.

Inside, 65-year-old dance instructor "Swingin' Lillian" Bradley, adorned in fluorescent yellow beads and matching oversized earrings, bellowed her welcome, "Let's see some hip movement here."

The 18 graying students, many in heels and sun dresses, fell into four rows on the plush green carpet and began to twist and swing to the beat of "Blame It on the Bossa Nova."

"Every senior citizen isn't just doing the hokeypokey or the bunny hop," said Bradley, a retired data processor. "We're way beyond that."

Bradley and more than 2,500 members of the over-60 set belong to an innovative program in Prince George's County called Older Adult Service and Information System, or OASIS.

The venture is housed at Hecht's and its $50,000 annual budget is funded by the store, the Prince George's County Department of Aging and HealthPlus/65, a private health maintenance organization.

OASIS offers a wide variety of educational programs geared toward energetic older adults who enjoy the arts and humanities and may not feel comfortable in traditional senior citizen centers. Membership in most programs is free.

Instructors for OASIS classes, workshops and outings include volunteers from the Smithsonian Institution, the University of Maryland and area community colleges.

"OASIS classes go in depth," said Reed Dewey, the program's former coordinator. "For instance, instead of offering a course in general current events, we give a class on federal issues and how they affect the senior."

Because of the quality of its programming, OASIS received an Outstanding Achievement award from the National Association of County Organizations this year.

Since the Prince George's facility opened in June 1986, several other OASIS centers have sprung up nationwide under the sponsorship of the May Department Stores Co., the St. Louis-based parent corporation of the Hecht Co., and under the guidance of educator Marylen Mann, the originator of the program and current executive director of the nationwide network of OASIS centers.

OASIS serves "middle income, wide-awake people," said Barbara Kohrn, a 66-year-old retired program analyst from the Department of the Treasury.

Kohrn, cochair of volunteers at the Prince George's center, added, "We deserve a little something at this point in life." The average age of OASIS members is 71.

Emerging from the dance lineup, Joe Graves, a retiree from the Government Printing Office and grandfather of 21, said he was originally attracted to OASIS because of its Sunshine Walking Club.

"You walk from 8:30 in the morning up and down the Plaza," Graves explained. "Of course we snitch on that some mornings and come a little later." The walkers tick off six or seven laps through the mall about three times each week.

"All the merchants and everyone were just very happy to have OASIS come into the store," observed Linda W. Botts as she sat in the office of the OASIS center.

Botts, who oversees the center for Prince George's Department of Services and Programs for the Aging, points out that Hecht's gives special discount days for OASIS members several times a year. "It's a mutually beneficial relationship and that's why it works."

Battling to be heard above the lyrics from the dance class, Botts added: "We feel that there are certain things that the private sector can do that we are unable to do. The ultimate goal, at least from the department's perspective, was to reach a segment of the population which we felt we were not adequately reaching."

Senior citizens who come to the mall "have an opportunity to come into an environment where they do not feel threatened," she explained. "Everybody loves to shop."

And seniors flock to classes at the OASIS center, most of which are free. "Sometimes you can't get a chair into the room," said one participant.

Workshops range from quilting and physical fitness to advanced investment strategy taught by an account vice president from PaineWebber, a Litton microwave cooking seminar and highlights of European history.

The history course, according to the OASIS brochure, is "ideal for those considering a trip to Europe."

"The principle of OASIS is magnificent," said C. Margaret Hall, a sociologist who is an authority on aging at Georgetown University, "since it offers older people a facility where they can continue to be active in later years and helps to create a new attitude toward aging as an opportunity for greater fulfillment."

Florence Werner, clad in a clinging tank top and bright lipstick, stepped out of the dance line.

"I don't really like line dancing," she said. "I do specialties . . . everything but break{dancing}." Werner lamented that she waited until age 62 to take up belly dancing.

A sporty, neatly dressed man who travels to the center from the District suggested that a similar program should be planned for Washington.

"You get a different class of people," he noted. "The majority of the elderly in D.C. can't afford a car. They need a place where they can relax and feel welcome."

As the advanced line-dancing students began to move to the soulful tune of "Montego Bay," the instructor stood against the wall letting the students recall the intricate steps.

"I plan on signing up," said Minnie Boon, a 66-year-old retired custodian for the Prince George's County Board of Education and a first-time visitor to the OASIS dance class. She explained, "It must be the Saturday night fever."