When New York's Mostly Mozart Festival has come to town each summer, one of its consistent appeals has been to feature prominent young conductors whose strengths are seldom heard here. That's how Washington learned three summers ago that Leonard Slatkin of the Saint Louis Symphony is a superb Mozart conductor. And so it was last night that the Indianapolis Symphony's John Nelson proved to be not far behind the standard Slatkin set.

Under Nelson's baton, the Mostly Mozart Festival Orchestra was playing at its most precise and assured--with a firm pulse and clarity of line and texture that it does not always produce for other conductors. The orchestra is not always the most polished of ensembles, but last night the music was full of bouyance and style.

It was very much to the conductor's credit that the finest playing came in the most challenging work, the Mozart 24th piano concerto in C-minor, with Garrick Ohlsson the soloist.

The later Mozart concertos constitute such an unbroken sequence of masterpieces--there is nothing comparable elsewhere in the concerto literature--that one hesitates to single one out from the others. But if there is a pinnacle, the C-minor is it. Beethoven thought so--and he spoke with authority. The range of mood, melodic inspiration and tonal lustre that Mozart packs into these three movements is extraordinary--as the work moves from the dark, troubled chromatic harmonies of the sonata form opening movement to the exultant, incredibly rich theme and variations of the finale.

The density and intensity of the concerto was splendidly realized by Nelson and the orchestra, especially in the winds. It was an interpretation in which the work's high drama rested more with the orchestra than with the soloist. By comparison with, say, Schnabel, Ohlsson leaned toward understatement, but he was not in the least bland. His work was full of pearly runs and delicate moldings. It made an appealing contrast to Nelson's conducting. The Schumann-esque candenza in the first movement was by, of all people, Brahms.

There was another concerto, too--the fifth one for violin, with Dmitry Sitkovetsky the soloist. It's a wonderful work, and it was a good performance, but after the C-minor concerto the work seemed anticlimactic.

The lengthy concert opened with Haydn's sunny Sixth Symphony, Le Matin ("Morning"). Textures and balances were so fine that, for once, the harpsichord was almost always audible. And at the close came one of Mozart's most Haydn-like symphonies, the 33rd, in a fine performance.

Nelson will conduct the orchestra again Saturday night, with cellist Lynn Harrell and pianist Bella Davidovich.