Sarah Caldwell's done it again. On Oct. 20, she and the board of her Opera Company of Boston bought the Savoy Theater, built 50 years ago as the Keith. On Nov. 1, the company gave the first of four performances of "Tosca" in the theater, having done miracles in the place in 10 days that no one else would have attempted in under a hundred.

Its 2,800 seats were filled for the last three performances, all the way up into the two soaring balconies where the acoustics are excellent, as they seem to be in most of the house. The original beautiful Italian marble columns stand like sentries along the walls while the Parisian crystal chandeliers and a grand foyer are deliberate reminders of the Parris Opera.

Now that the "Tosca" performances are out of the way, Caldwell's plans call for closing the house for a three-months' renovation. For years the stage area had served as a second movie theater. It was re-opened for "Tosca," but backstage areas still have to be restored to the dressing rooms they once were instead of the 40 single apartments-for-rent they had become. The orchestra pit, designed for a vaudeville band of around 30, will be enlarged to operatic proportions; and the back wall of the stage is slated to be moved back another 20 feet, thanks to assistance from city officials in closing off an adjacent alleyway. Solid backing from Mayor Kevin White is evident in much of Caldwell's planning.

The theater is basically in excellent shape. It was, at $850,000, a rate bargain. Among the new doors it will open for the indomitable Caldwell are opportunities for more children's opera, for an education branch where the operatic arts and administrative details can be taught, and space for the company's business offices which have, up to now, been rented at some distance from wherever the company might be playing.

As for the "Tosca," it, too, was full of unusual drama. Magda Olivero sang the title role as she will do later this season in San Francisco and at the Metropolitan Olivero is, by accurate accounts, 68, an age at which few sopranos can croak at all. (Her debut was in 1933 in "Gianni Schicchi.")

Hers was the finest acted "Tosca" in my experience, and almost entirely satisfactory, often of rare beauty in song. She had every note of the score where she wanted it. To see this fabled woman crossing the stage on her knees to plead with Scarpia was to believe that age can indeed be made relative.

With Fred Scott conducting at 25, surely the age spread between conductor and prima donna set an all-time record. He gave her brilliant support, clearly aware of precisely where she wanted more volume, a more rapid tempo, or time to revel in a tone.