Dr. O'Neill grew up outside of Ann Arbor, MI, and attended Michigan State University. She pursued her veterinary education at Ross University in St. Kitts, followed by a clinical year at the University of Missouri. Sydney completed a small animal rotating internship and a small animal surgical internship at Michigan State University. Dr. O'Neill joined Metropolitan's team in July 2023 to continue her surgical internship training before accepting a residency position in July 2025.

Outside of the hospital, Dr. O'Neill enjoys spending time outside, visiting her family in Michigan, and spoiling her 2 cats (Weasley and Barbara Jean).