Two years ago, Mary O'Shaughnessy was toiling in a Chicago dark room as a photographer's assistant, developing pictures for camp "memory books." She rolls her eyes upward when she thinks of it now.

"It was more fun than working," she recalled today. Now she's in the business of owning and riding horses as an amateur. And business is booming.

O'Shaughnessy made her amateur debut this week aboard the Silent Step, a 4-year-old steel-gray mare, and took second place in the amateur-owner hunter division at the Middleburg National Horse Show.

That was fine for beginners, but Silent Step is on her way as a strong contender for the hunter's stakes title Thursday after picking up three firsts in the first-year green working hunter division today with rider-trainer Ira Zimmerman aboard.

Zimmerman, 27, who comes from a family of professional horsemen in Glenelg, Md., met O'Shaughnessy, 26, in her native Chicago and has been riding Silent Step for her in the nine shows the horse has entered this year.

Silent Step has won 10 classes in the last three weeks, including the Virginia State Fair championship, the Winston-Saltem championship and the three hunter divisions here.

O'Shaughnessy, who has mored to Charlottesville where Zimmerman also operates a horse farm, has been riding since she was six years old and interrupted her riding only when at Barat College.She majored in technical theater, studying set and costume designs.

O'Shaughnessy said her family, and the Zimmermans has always had show horses. "It was the way we made our living," said Zimmerman, who with his four brothers trained and showed horses for his father.

"You're lucky if you can take a horse around (the circuit) for $20,000 a year. You go from February in Florida up through the indoor circuit in October. We plan to take her (Silent Step) to the Washington International in October at Capitol Centre."

The horse show at Foxcroft School on Rte. 626 off Rte. 50 ends Thursday with the grand finale of the Hunter Classic, the Junior Amateur classic and the Ladies Sidesaddle. Preliminaries begin at 9 a.m.

GRAND JUNIOR HUNTER - Champion. Escaride. Tracey Wembers: reserve. The Graduate. White Oak Farm.

SMALL JUNIOR HUNTER - Champion. Escadride. Tracey Wembers: reserve.Chapeau D'or, Freddi Baldi.

LARGE JUNIOR HUNTER - Champion. The Graduate. White Oak farm; reserve. Hopefully Happy, Lindsey Ann Evans.

SMALL HUNTER - Champion. Bold Acquisition, Lisa Stone; reserve. Without A Doubt, E. L. MacDonald.

AMATEUR OWNER - Champion (section a), Beachboy, Elaine Cone; reserve. Lemon Springs. Sanora Caldwell.

AMATEUR OWNER - Champion (section b), Snickdefritz, Mrs. R. J. Reynolds; reserve, Moon Shadow, Sue Sherwood.

FIRST YEAR GREEN HUNTER - Champion, Silent Step, Mary O'Shaughnessy; reserve, More Like Magic, Julie Kennedy.

SECOND YEAR GREEN - Champion, Random Harvest, Rosenmount Farm; reserve, First At Line, Mark Smith.