Pianist, composer, pop/classical crossover whiz, famed reinterpreter of Radiohead music and popular host of the NPR radio show "From the Top," Christopher O'Riley will be the guest soloist for the San Francisco Girls Chorus' season-closing concerts Friday and Saturday at the San Francisco Conservatory of Music. He'll be bringing with him arrangements he has recently composed of four Joni Mitchell songs from the "Song to a Seagull" album, which will get their world premiere on the concerts.

The Chicago-born O'Riley, 56, and the five-time Grammy-winning Girls Chorus had their first encounter when they were his guests several years ago on the weekly radio show that hauls aspiring young talent from all over the country and presents them on air. Their collaborative concert, called "Truth and Beauty," will also include Vaughan Williams' "Let Beauty Awake," Claude Debussy's "Salut Printemps," Eric Whitacre's "Five Hebrew Love Songs," Stephen Paulus' "Sing Creations Music On," works by Gustav Holst, and more.

Founded in 1978, the Girls Chorus is now in transition with the nonrenewal last year of longtime director Susan McMane's contract. Working with them this spring, and preparing this final concert, is guest conductor Brandon Brack, a former member of the Bay Area's well-known male a cappella ensemble Chanticleer. Brack has been named interim director for the Chorus in its next season, which opens Nov. 12 in the Herbst Theatre and will feature Berkeley native Gabriela Lena Frank as composer-in-residence.

This weekend's programs take place at 8 p.m. Saturday and Sunday at the Conservatory, 50 Oak St. in San Francisco. Tickets, $30-$35, are at 415-392-4400 or www.cityboxoffice.com.

A LEG UP FOR EMERGING COMPOSERS: Michael Tilson Thomas, music director of both the San Francisco Symphony and the Miami-based New World Symphony, and music publishers Boosey & Hawkes have a collaborative project going which will give promising young composers from across the Americas a decided boost. The first beneficiary of their New Voices initiative is Canadian-born, New York-based composer Zosha Di Castri. A doctoral candidate at Columbia University who has also studied in Paris, she has already won several composition prizes, had a work premiere at last summer's Cabrillo Festival in Santa Cruz with Marin Alsop conducting and is serving as composer-in-residence for the new Ensemble Portmantô in Montreal. Four New Voices panelists -- Tilson Thomas, Berkeley composer John Adams and composers Steven Stucky and Steven Mackey -- have just chosen Di Castri to write two new commissioned works, one a chamber piece and the other for full orchestra. She will have extensive contact with and support from both the New World and San Francisco symphonies as these pieces develop and will get technical training and advice from Boosey & Hawkes on the business end of a composer's job. Both of her pieces will premiere in Miami in April and then be played by the San Francisco Symphony on its regular season lineup and its chamber series the following fall.

RACH TO ROCK THE HOUSE: And speaking of the San Francisco Symphony, the orchestra has graciously vacated Davies Hall for this weekend to accommodate the two-day visit of Charles Dutoit and the Philadelphia Orchestra -- the last of the six high-caliber American orchestras to pay court during the Symphony's centennial season. You can find Philadelphia's plans for the weekend listed among our Good Times enticements in the first few pages of TimeOut Weekend.

But MTT and the gang will be back in their proper places June 14-17, and they'll have a wonderful soloist in tow. The fabulously gifted Yuja Wang will be at the keyboard to facilitate everyone's wallowing in the sweeping romanticism of the Rachmaninoff Piano Concerto No. 3. Also on the program is Jean Sibelius' vintage-1907 Symphony No. 3, a watershed in the Finnish composer's oeuvre that marked a swing away from the lush romanticism of his first two symphonies and a march toward the more ethereal austerity of his later works. Faure's "Pavane" will open the concerts, 8 p.m. June 14-16 and 2 p.m. June 17. Tickets, $15-$145, are available at 415-864-6000 and www.sfsymphony.org.

Contact Sue Gilmore at sgilmore@bayareanewsgroup.com.