What does a fashion designer's assistant do after nine years of working with one of the best? Become a designer herself. That's what 33-year-old Carolyne Roehm did after working on Oscar de la Renta's spring line last November. She struck out on her own, and in April introduced her first collection in New York to rave reviews.

Roehm made her first public appearance as a designer at Garfinckel's last week. "I had the most wonderful job working with Oscar, but I didn't want to be the world's oldest assistant," she told the guests at a luncheon in her honor given by Garfinckel's president Hanne Merriman. "There was no future for me. Oscar is a relatively young man, and I wanted to be boss."

The collection was modeled informally during the lunch for such guests as Betty Lou Ourisman, Countess Monica Augusta, Princess Pahlavi, Analida de la Guardia, wife of the Panamanian ambassador, and Astridur Anderson, wife of the Icelandic ambassador.

The collection includes suits, dresses for day and cocktails, but the emphasis is on evening elegance. Many of the slim gowns are strapless, some topped with bolero jackets. Velvet bodices are embroidered with beads to look like jewelry, and printed silk cloque' is draped close to the waist.