Open for lunch Monday through Saturday 11:30 a.m. to 3 p.m., dinner 5:30 p.m. to 10:30 p.m. A. E., MC,V. Reservations.

Prices: at lunch, main dishes $3.95 to $8.25. Appetizers at dinner $2 to $6, main dishes $7.50 to $15.50. Average dinner with wine, tax and tip about $25 a person.

In Moslem Iran a restaurant would not serve lobster. But Omar Khayyam, the new incarnation of a very fine Alexandria restaurant that closed a little over a year ago, has assimilated this time around. Less than half its menu is Persian, the rest being nothing more Eastern than pate' and French onion soup, duck with orange sauce and veal Oscar. And at last check, its chef was American.

Still, its food, particularly its Persian food, is generally very good, and the dining room is every bit as attractive as a Persian painting.

The old Omar Khayyam was vertical, with small rooms on several floors, but this new one is a great expanse of space with not even columns to interrupt it. The walls give off a rosy glow, particularly the arched bar, which looks like a palatial cave. Walls and ceilings are decorated with plaster curlicues; colored glass set in the walls may not look Persian but is festive nevertheless. The chairs are velvet bcked, aand on each table is a hurricane lamp and a single rd rose. For a restaurant in office building environs, it is immensely pretty.

The waiters, on the other hand, are more authentic but less successful. Service is sometimes gracious, sometimes stern and often absent altogether. One night we ordered an appetizer of oysters with mushroom caps. The waiter brought snails, with the explanation that the chef thought we would like them better. When we questioned him further he said that the oysters were canned. Although it was thoughtful of them to refuse to serve something we might consider inferior, we wanted to choose our own substitution rather than have the dish brought unbidden. And so the waiter willingly exchanged the snails; since our next three choices were unavailable, we settled for soup. We have found the service unpredictable, sometimes helpful and sometimes moody and irritable, which is my hestitation about recommending Omar Khayyam, an otherwise extremely pleasant restaurant.

Choosing the food is easy: stick to the top -- the Persian -- offerings in each category. That means eggplant sauteed' to a crusty surface and topped with tomato-meat sauce, yogurt and mint, much like the Afghan version of eggplant; spinach with yogurt and dill, though this dish could take a lesson in zestiness from the Caspian Tea Room, as could the cucumbers with raisins, nuts and mint; annd very good sweet-and-lemony stuffed grape leaves. Soups are mild, freshly made, appetizing. The rest of the appetizers are Western, but then most of them have not been available when I ordered them anyway.

The list of entrees begins with eight Persian main courses, most of them kebabs, of chicken, sliced lamb or beef, ground lamb. All have been mildly seasoned and beautifully cooked, tender and moist. Lamb chops are more pungent, thin, crisp and delicious except for one that tasted as if it ere past its pull-date. Some of the more complex Persian entrees occassionally showed more mixed success. For a wonderfully exotic combination of tastes, try Baghali Polo, rice tossed with lima beans and a lot of dill, served with a tender sliced lamb shank. My favorite Persian dish, Khoresht Fessenjan, was the most disappointing, for this chicken with walnuts and pomegranate juice was overly sweet, lacking tartness and balance.

The highlight of a Persian meal is likely to be the rice. Here it is infused with butter, is soft and pure white and is contrasted with bright yellow grains of rice sprinkled on top. But unlike the Caspian Tea Room and the Stop Inn, Omar Khayyam does not offer powdered sumac to sprinkle on the rice, nor does it bring egg yolk to toss with the rice and sumac. Ask for them or you will miss one of the best reasons for dining Persian style. Also, one night our entrees came with delicious buttered and herbed zucchini, but otherwise rice was the sole accompaniment, except for a light and spongy homemade bread, cut in squares, nice touch. The menu goes on, through veal, "Chicken Coq au Vin," duck, salmon and lobster tails; the veal has been good quality, cooked skillfully, with a creamy and rich sauce, but less distinctive than thhe Persian dishes.

The wine list is more notable for its original spellings than for its original choices, but good value is available in the '80 Simi Chenin Blanc at $8.75.

Dessert meanders into peach melba and Haagen-Dazs ice cream, hardly traditional Persian. But there is baklava that was acceptable one day and soggy another, and a pretty good strawberry shortcake. The finish, as to be expected, has been Turkish coffee.