Ms. O'Reilly, best known for her role in last summer's Academy Award-winning movie "The Help," returned to the school where she graduated in 2003 to discuss her experiences as an actress.
She credited her friendship with Octavia Spencer — who won the Oscar for best supporting actress as the outspoken maid Minny in "The Help" — for getting her an audition that led to her first role in a major motion picture.
"To get into a room like that is incredibly difficult, and only something you can do if you have a big powerful agent, which I didn't have," Ms. O'Reilly told more than 200 Menlo students who packed a school lecture hall during their Monday lunch hour.
"Octavia snuck me in because she was friends with the casting director."
She said she and fellow cast members studied the 1990s PBS civil-rights series "Eyes on the Prize" and the documentary "Yes, M'am" to prepare for their roles in "The Help," an adaptation of Katherine Stockett's 2009 novel about black maids working for white families in Jackson, Mississippi, in the 1960s.
Of her character, the anxious, socially aspiring Elizabeth Leefolt, she said "many would say she was a despicable woman, but it was up to me not to judge her so that I could portray that woman. It was trying to look at that human being from a different angle and see what in them is human.
"In that case, I hugely identified — and I think anyone in this room can identify — with just desperately wanting to try to fit in."
Ms. O'Reilly, who took acting lessons in San Francisco during her high school years, said she has aspired to an acting career since the age of 8, never considering anything else.
After Menlo, she attended the University of Southern California for a year before leaving to pursue acting, where she found small television and movie roles including a movie called "Herpes Boy," in which she met Spencer.
One of her biggest challenges in the business, she said, is staying motivated and productive between jobs.
"I really don't have a typical day — they're annoyingly all over the place," she said. "Some days it takes a lot to just not sleep in and watch TV. When you're not on a job you just have to keep going to class and working on those emotional and technical muscles."
She counts herself lucky to have settled early in life on a career she loves.
"If you know what you love and it doesn't seem like a practical thing to do, it's worth examining, but that passion you have could turn into hard work and that hard work could turn into a successful career as an actor, or musician or poet," she told the students.
"If you don't love what you do, you might succeed in certain ways but you won't be a success."
She spoke with student interviewer Samantha Bergeson in Menlo's monthly, student-led Coat of Arms Speaker Series, whose previous guests have included former U.S. Secretary of State Condoleezza Rice and former CIA director James Woolsey.