Tonight’s concert by the Boston Symphony Orchestra may be one of the last opportunities to see conductor Seiji Ozawa perform with the acclaimed group in D.C. Ozawa, who has had the longest tenure of any conductor in BSO history and is also one of the longest-serving conductors of any of the world’s major orchestras, will step down from the podium in 2002 to assume the music directorship of the Vienna State Opera. The shift will leave vacant a much-coveted position, and the question of Ozawa’s successor is a hot topic among the classical-music set. The variety of the program this evening is a testament to Ozawa’s versatility as a director, ranging from Debussy’s coloristic “Nuages” from Nocturnes to Lutoslawski’s rigorous neoclassical piece Concerto for Orchestra, and also including one of the staples of the symphonic repertoire, Brahms’ Symphony No. 3. At 4 p.m. at the Kennedy Center’s Concert Hall. $20-$75. (202) 467-4600. (Amy Domingues)