O'Donnell attended high school at The Bromfield School, which has a notable drama department, and he was involved in numerous productions both on the festival circuit and in community theater. In 1996 he was awarded a Massachusetts High School Drama Guild Acting Scholarship. Upon graduation he joined the first class of the newly created four-year classical theatre acting conservatory at The Hartt School in Hartford, Connecticut. The Hartt School of Theatre was established by Malcolm Morrison and Alan Rust (North Carolina School of the Arts). At the Hartt School he performed in numerous plays including Three Sisters, Lysistrata, Twelfth Night, Philadelphia Here I Come, and played Romeo in Romeo and Juliet opposite Hannah Mello. He graduated in 2000 and was accepted to the Hartford Stage Co. Regional Theatre, where he appeared in Macbeth and A Christmas Carol. In early 2001, he moved to Los Angeles to pursue work in film and television. In the fall of 2001 he was cast as a lead in the US premiere of The Man Who Never Yet Saw Woman's Nakedness by Moritz Rinke, at The Odyssey Theatre in West Los Angeles.