Kelley O’Connor doubts she will ever perform at the esteemed Metropolitan Opera, and the mezzo-soprano isn’t concerned about that absence on her resume in the slightest. Although she does sometimes perform in operatic productions, for instance, her upcoming March debut in Benjamin Britten’s The Rape of Lucretia at the Boston Lyric Opera, most of her career success has come on the concert stage.

That’s exactly where Chicago listeners will have a chance to hear her Oct. 11-14, when O’Connor joins guest conductor Andrés Orozco-Estrada in performances of Mahler’s Symphony No. 3. Although she started her career with a focus entirely on new and more offbeat repertoire, she has put greater emphasis in recent seasons on more mainstream works, especially those of Mahler.

“I’ve been lucky because I’ve been able to go back a little bit and explore this step that I skipped when I went right into new music,” she said. “So it’s been really great that I can balance doing the new music with the traditional music. It still has been primarily concert [works], which is what I love. It still is and I think it always will be. I really, really prefer not having costumes or a set or any of that. Being on stage with an orchestra, for me, feels like a much more collaborative experience, and I feel much more a part of the whole in that situation than I do on an opera stage. It’s been to my benefit that I’ve been able to do my career the way I wanted to.”

Although she has performed in Mahler’s Symphonies Nos. 2 and 3 for virtually her entire career, she has only recently expanded her reach in the composer’s repertoire. Two years ago with the Indianapolis Symphony, she made her debut in his Des Knaben Wunderhorn, settings of German folk songs. “I feel like they are perfect for me,” she said. At the suggestion of Donald Runnicles, music director of the Grand Teton Music Festival in Jackson, Wyo., she took her first solo turn there three years ago in Mahler’s 1909 song symphony, Das Lied von der Erde.

The genre-busting set of six orchestral songs, which are based on German free-verse translations of Chinese poems, represent a profound meditation on life and death and a culmination of everything the composer had done before. “That was a really big moment when I was finally able to sing that,” O’Connor said, “because I don’t think I ever considered myself ready to do that.” This season, she will perform Das Lied during two more sets of concerts with Runnicles: in January with the Dallas Symphony Orchestra and in April with the BBC Scottish Symphony.

Completed in 1896, Mahler’s Symphony No. 3 is his longest symphony, with some performances clocking in at more than 100 minutes. The vocal solos do not come until the fourth and fifth movements. Indeed, audience members have asked O’Connor about the challenge of sitting still on stage for much of the work. But she said that she enjoys listening to everything that comes before, and it is important that she, like the listeners, experience the full scope of what the work represents. Before the symphony was published, Mahler gave the programmatic work’s evocative movements titles like “Pan Awakes, Summer Marches In” and “What the Animals in the Forest Tell Me.” “It’s my favorite [Mahler work],” she said. “I love it so much. You get a complete glimpse into his person. It’s the full journey of Mahler.”

O’Connor is particularly taken with the final movement, which has no singing. Mahler marked it Langsam — Ruhevoll — Empfunden (slowly, tranquil, deeply felt). “The last movement to me is kind of everything,” she said. “It’s just so magnificent and perfect that I love experiencing it every time.”

The mezzo-soprano has worked with Orozco-Estrada several times, including December performances of Mahler’s Eighth Symphony with the Tonkünstler Orchestra in Vienna and St. Polten, Austria. “I love him,” she said. “He’s so brilliant,” she said. “He’s from Colombia but he trained in Vienna. He speaks all the languages, and he has training from everywhere, so he can tap into so many different aspects of what musicians need. I’m so excited that I get to perform Mahler Three with him there.”

O’Connor has a rich history in Chicago, especially with the CSO. She made her debut with the orchestra in Feburary 2008 in concert performances of Osvaldo Golijov’s opera Ainadamar. Later that year, she joined the orchestra in Chicago and Carnegie Hall for her first performances of Neruda Songs, by composer Peter Lieberson. He wrote them in 2005 as love songs for his wife, famed mezzo-soprano Lorraine Hunt Lieberson, who died a year later. In 2009, O’Connor was a soloist in the orchestra’s performance of Beethoven’s Ninth Symphony at the Ravinia Festival, and more recently, she joined guest conductor Michael Tilson Thomas in Stravinsky’s Elegy for J.F.K. O’Connor also has sung with Lyric Opera of Chicago and made her debut his past summer with the Grant Park Music Festival.

So, no, O’Connor has never sung at Metropolitan Opera. But among aficionados of refined classical singing, her bewitching voice and nuanced expressiveness is highly regarded.

“I don’t think I could have chosen any better route for myself,” she said. “The doors have kind of been opened for me because of the way I chose to live this career and how we started it. I met the right composers, I met the right conductors at the right time.”