Oboist Ronald Roseman's presence added a special flavor to last night's concert by the Juilliard String Quartet at the Library of Congress. In addition to the engaging Mozart Quartet in F, K. 370, for oboe and string trio -- almost certain to turn up whenever this combination gets together -- the evening included a little-known work written for those same instruments by Benjamin Britten when he was only 19.

Entitled "Phantasy Quartet" and designated Op. 2, this youthful effort is a delight. It begins softly with a whimsical cello motif, which the other strings take up in an impudent exchange. This rhythmic horseplay continues as the oboe comes soaring in with a lovely melody directly inspired by the British folk tradition. Imaginative and appealing despite a certain unevenness, the music points directly to the mature Britten style by immediately winning the listener's heart.

In both obow quartets Roseman revealed an exceptional tone control and a pleasing musicality. He reached easily and smoothly for high notes, producing a mellow sound that fell gently upon the ears. The piercing, nasal quality frequently associated with the oboe was strikingly absent.

As part of the Library's ongoing observance of the 100th anniversary of Ernest Bloch's birth, two later works by the composer completed the program. His fourth string quartet, a powerful piece rich in interior explorations, was performed with appropriate intensity by the Juilliard. Joel Krosnick's passionate playing of the demanding Suite No. 1 for solo cello proved equally impressive.